I just tried to follow 1000 accounts - including all the ones you follow - but there seems to be a limit and I got stuck at 740 so will try again later
PS - did the rest the next day - made very little difference to number of followers so I don't know why people do that - takes ages to click on them all!
I have no idea why neither, it just clogs the feed up, so you can not see the posts and people you want to see, I never did it, I did make the mistake of following every single person that ever talked to me at one point, then deleted most of them.
Quality over quantity. :-)