Dumbledore knows that the time will come—hopefully much later, but eventually it will come—for Harry to face off against Voldemort. Dumbledore’s goal from the minute Harry steps into Hogwarts is to give Harry his best fighting chance—because according to the prophecy, Harry is the wizarding world’s best chance to vanquish Voldemort. Dumbledore wants Harry’s first attempt at facing off against Voldemort to happen in as controlled an environment as possible. And such an opportunity falls into Dumbledore’s lap just as Harry is coming to Hogwarts!
Dumbledore knows that “Quirrell, full of hatred, greed, and ambition, sharing his soul with Voldemort, could not touch [Harry]” because of Lily’s sacrifice (SS299). So this would have been as good a trial run as was possible: Harry would face off against Quirrell, he would emerge victorious because Quirrell cannot touch him, and that will be that. The Stone’s not in danger; Harry isn’t in too much danger. Not only will this give Harry experience, it will also be an important confidence booster to a kid who’s been downtrodden most of his life.
In fact, that’s the more crucial aspect: going forward, Harry needs the confidence of having faced off against Voldemort. He needs to not lose his head when he comes face to face with Voldemort. Indeed, by the time Voldemort returns to his body, Harry is an old pro at facing him—he’s faced Voldemort more often than he’s taken exams!
Admittedly, Dumbledore probably wished this whole Voldemort-fighting dress rehearsal would happen later than Harry’s first year, but he is quite adept at pivoting and changing plans as necessary. An important bit of perspective to always maintain is that the characters don’t know they’re in a seven-book series. Dumbledore explains to Harry (again in that invaluable Order of the Phoenix confession), “You rose magnificently to the challenge that faced you, and sooner—much sooner—than I had expected, you found yourself face-to-face with Voldemort.” (OP837)
This refers to Dumbledore’s expectations before all this Quirrell stuff went down. When Dumbledore got wind of what was going on, he moved up his timeline and set up the showdown for Harry’s first year.
This same reasoning explains the presence of protections two through six. At first glance, why bother with all the “protections” surrounding the Stone? Only two of them were even remotely effective: Fluffy and the Mirror of Erised. The rest of them—Sprout’s Devil’s Snare, Flitwick’s flying keys, McGonagall’s chess board, and Snape’s riddle—are gotten through by a trio of first years and don’t serve to hinder Quirrell at all.
Dumbledore tells Harry, “I have watched you more closely than you can have imagined.” (OP839) But along with watching Harry, he watched Ron and Hermione, once it became evident that they would be Harry’s closest friends and staunchest allies.
It’s made clear in the latter books that Dumbledore has been watching Harry’s friends. He tells Harry in Half-Blood Prince, “I think you ought to relax [the decision to tell no one about the prophecy] in favor of your friends, Mr. Ronald Weasley and Miss Hermione Granger.” (HBP78) He also gives Harry permission to tell Ron and Hermione about his forays into Voldemort’s past via Pensieve, saying, “I think Mr. Weasley and Miss Granger have proved themselves trustworthy.” (HBP215)
It’s easy to dismiss this, but Dumbledore is perhaps the least trusting character in the series. He gives his closest allies the absolute bare minimum of information necessary and does not wholly confide in anyone. Yet he instructs Harry to reveal to Ron and Hermione things that are known only to Harry and Dumbledore himself. This is a big deal. Dumbledore would not place such trust lightly, so this is evidence that he knows a great deal about Harry’s friends.
We also see this come into play when he leaves both of them items in his will, despite making “exceptionally few personal bequests.” (DH124) He gave Ron the Deluminator, knowing that Ron’s commitment to the Horcrux hunt might waver (DH391). He gave Hermione The Tales of Beedle the Bard to decipher and trusted her to combat Harry’s rash nature (DH720).
But back in Sorcerer’s Stone, Dumbledore did not yet know if Ron and Hermione would be the staunch allies that Harry would later need. He needed a way to cement their collaboration for the future. And what better way than to have them help Harry get to Quirrell? So he decided to put in some barriers specifically for the trio to work their way through. This gives them valuable experience working together through magical obstacles as well as confidence in having triumphed. And this is the reason for protections two through six, tailored specifically to their strengths—to establish the Trio as we know it.
An interesting theory I’ve come across, lately in Lorrie Kim’s SNAPE: A Definitive Reading (p. 18), is that Dumbledore’s original intention was for Neville to also be part of the crew that went through the trapdoor. Ergo, the Devil’s Snare was an obstacle meant for him, given his strength in Herbology. It’s a good theory but does not really gel with how meticulous Dumbledore is about these things.
Neville’s proficiency at Herbology is not as overt in Sorcerer’s Stone; the first time it’s mentioned is Neville getting a “good Herbology mark making up for his abysmal Potions one,” (SS307) in the closing pages of the book. Hermione’s aptitude for fires seems to be more indicative of facing off against Devil’s Snare.
More importantly, if Dumbledore watched Harry’s friendships as closely as we believe, he would have known Neville would not be included. The only times Neville is together with the Trio is watching Quidditch matches, during the late-night Norbert fiasco, and in the ensuing detention. Not exactly bosom friends, and Neville’s being misinformed about Norbert further indicates lack of confidence. The Devil’s Snare is the first obstacle after Fluffy; Dumbledore could have gone at any point and removed it once he figured out
