Boggarts turn into their victim's worst fears. Why do they do this? What is their ultimate goal?
It should probably be mentioned that likely the Boggart was Rowling's attempt to provide a real world explanation for "boogey men" and monsters in the closet, or whatnot, as the first book was very much meant to be read by young readers.– NeilSep 27, 2018 at 8:49
Boggarts feed off of fear and grow stronger from it.
Boggarts feed off of fear and grow stronger from it. Therefore, the reason they’d want to create fear would be so they could feed off of it. In the J.K. Rowling writing on Pottermore about Boggarts, it’s mentioned that Boggarts seem to be created and sustained by human emotions.
Boggarts can be made to disappear, but more Boggarts will inevitably arise to take their place. Like poltergeists and the more sinister Dementors, they seem to be generated and sustained by human emotions.
- Boggart (Pottermore)
The case of one particular Boggart, the Screaming Bogey of Strathtully, further shows that Boggarts grow stronger by feeding on fear, as that particular Boggart had fed on the fears of Muggles to such an extent that it could become a large black shadow.
Famous Boggarts include the Old Boggle of Canterbury (believed by local Muggles to be a mad, cannibalistic hermit that lived in a cave; in reality a particularly small Boggart that had learnt how to make the most of echoes); the Bludgeoning Boggart of Old London Town (a Boggart that had taken on the form of a murderous thug that prowled the back streets of nineteenth-century London, but which could be reduced to a hamster with one simple incantation); and the Screaming Bogey of Strathtully (a Scottish Boggart that had fed on the fears of local Muggles to the point that it had become an elephantine black shadow with glowing white eyes, but which Lyall Lupin of the Ministry of Magic eventually trapped in a matchbox).
- Boggart (Pottermore)
Therefore, the reason that Boggarts turn into people’s worst fears is so they could create the largest amount of fear possible so they could have more fear to feed off of.
6I would also specifically add that boggarts are non-living entities. As such, they have no specific goal, which requires sentience. They simply do as they are created to do by our own human psyche: scare us. Sep 25, 2018 at 16:37