Popular users do that so that their meme reaches the fun front page. I think you need 15 upvotes for it to appear there then it'll reach hundreds or thousands of upvotes even if it was a lame meme
Well I've seen it before you need 15 upvotes for your meme to reach hot. When it does reach hot then you'll get lost of upvotes. Well they arnt exactly popular but they are the kind that care about points. Unlike most of us. It's kinda annoying making alot of meme plugs so I'm not defending them I'm just saying why they do it.