Theres lots of reasons.
Some want to play with legitimately new friends and don't want to have their main accounts MMR result in completely unfair matches for their new friend. If they exclusively play with their new friend(s) then their new account(s) won't get particularly high MMRs. (Plus they don't always try very hard to win, it's more they won't feed and will only get fed enough to ensure they can pull their friend out of a bad spot as they focus on their friends laning/csing/etc instead of really paying attention to their own game.)
Playing things other than their main can be super tricky, as many people play their main/role champ at a much higher level than champs/roles they're learning early on. Diving into a role you haven't played since you were bronze when you're matched with plat/diamonds is going to be an ugly stomp. And even if you get stomped enough to drop your mmr down to a level you can play your off-role at, then it turns into a stomp when you do play your main role.
Some people insist ELO hell exists, and occasionally more experienced people want to confirm that that's just a thing bronze level players say due to denial. So higher ranking players make new accounts, intentionally get placed bronze, and then climb out. Similarly theres lots of bronze->diamond video series that youtubers will do, generally taking less than a month.
Others still like to do hard main accounts, where they only play 1 champion and that it. If that champs main role is taken, then they'll still use it in unconventional roles, so you might e.g. have janna adc.
Some people do just want to stomp new players. Their accounts will get high mmr's relatively quickly, but then they'll just make another one.
Having secondary account(s) to keep private can also be appealing to some people who may want to just play on their own without a bunch of their friends bugging them to play. They might legitimately be friends, but sometimes people aren't in the mood but don't want to hurt feelings. This is also relevant for internet personalities who might have a certain persona they prefer to maintain officially, but might conduct themselves a bit differently when just doing their own thing.
Theres also aram only accounts, where they'll only buy champs that are strong in ARAM in order to bias their chances of getting relatively OP aram champs.
I'm sure theres more reasons I can't think of right now, but theres certainly some legitimate reasons why people would want multiple accounts that aren't at the expense of others.