If you see no reasons why someone would switch accounts, then you just haven't really thought it through. And you do people a serious disservice by painting all of them who use multiple accounts with the same brush.
For one thing, people treat comments by moderators differently then they treat comments by everyone else. So if a moderator wishes to contribute a point of view to their own sub, but don't want people to assign it special weight, it makes sense to post it under a non-moderator account.
And if you don't understand the importance of sex in the human experience, then that's something you'll have to figure out on your own. But consider that without an interest in sex, you would not be alive.
There's nothing wrong with desiring sex and having sexual attraction to others. It's literally the genetic hardwiring that is responsible for the procreation and survival of our species as well as many others.
It's not uncommon for people to have one account configured to show NSFW content and another account used at work that does not - for obvious reasons of professional behavior.
And for those who are NSFW content creators, they have an even more obvious reason to separate that side of themselves from other discussions.
Another reason why someone would have separate accounts is to have one account where they expose their profession in order to be able to contribute as a subject matter expert, and another account where they participate in discussions without that link to their life. So a lawyer might use one account to offer legal opinions and another account to talk about their gaming hobbies without being constantly asked for legal help.
Just because you, personally, don't feel a need to have multiple accounts does not mean that there is no good reason for others to do so.
I can explain it to you, but I can't understand it for you.
Have a good day.