It's usually by/attracts people who are desperate for numbers and/or have heard that having numbers makes it easier to get more numbers without understanding why (the actual why is because if you have higher numbers likelyhood is that the people who read your stuff will also recommend your stuff to their friends and interaction boosts you in the algorithm... and empty subs do neither of those things.)
I can kind of understand them though, it can feel very lonely if you put a lot of work into something that you feel like no one is seeing, especially if you have jealousy blinders on and you're constantly looking at more successful comics going "my story's better than this, why won't anyone see it?" and if that goes on long enough I totally get why some people resort to sub4sub. Plus they can constantly see other people around them going "My comic hit 250 subs in under a year" "my comic hit 250 subs in under 3 months" but they can't see the hours of work behind those numbers and it's so easy to become bitter when you've been here longer working your arse off for less.
Tapas Interface doesn't really help either, with the sub counter on the dash and the promise of monetisation at 250 subs... even though I feel like it's not a secret that if you have subs but no one is actually reading you're going to earn litteraly nothing, some people still haven't really thought through the maths on things like adrev enough to realise even a dollar per 1k views is an astronomically high expectation and reality is nowhere near that... and they're not even at 1k views total yet.
I don't really want to judge them, whether they be blissfully optimistic or bitter and desperate, but they are, in any case, doing something that is counter productive.