only used 1 account for the first 2 or 3 years. eventually got a second account since I was a HS miner at the time, and having someone who could haul my jetcans for me was a huge efficiency increase. dipped my toes into Faction warfare as well shortly after that, and back in the day off-grid boosting was broken AF so I used it for that as well. did that for a year or so, then dropped back down to 1 account for the next 4 ish years since I was a bit burned out, wasn’t logging in as much (was mostly afk for that time, just logging in for training, casual industry jobs, and every so often would check out anything new that came out)
2 years ago or so, when I “came back” and moved out to null, I started with the second account just to act as a hauler. then I stripped all the indy skills from my main and injected a dedicated industry character. actually stated making the kind of IG income that would let me support multiple accounts, and 13 accounts later (with 10 more currently training as alphas) you could say things have sorta snow-balled.
as for why I do it, some stuff scales really well with multi-boxing, mining particularly. and having the extra industry slots speeds up large builds immensely. But mostly what I enjoy about it is the versatility, just about anything I feel like doing that day, I probably have an alt for that. if I feel like relaxing in a belt, i’ve got rorqs and barges, if I want to roam low-sec, I got a sub-cap specialist for that, if there is a big super-brawl, got one of those as well. all this without having to sacrifice training time on my main to branch out.
not to mention the pure convenience of being able to do things with alts on another screen while you focus on something with your main. just yesterday I was chaining a bunch of abyssals and was on an absolute roll, but I decided I wanted something from jita to improve my fit, so I logged in my jita alt, bought the stuff, threw up a courier contract. while one of my other alts ran to the local hub to grab some more boosters and ammo since I was running a little low.
For me its about “fun per hour” the more alts I have, and the more varied their skillsets are, the more likely it is that i’ll be able to have something I feel like doing that day, so I spend less time ship spinning.