Same reason I buy albums that I love despite me already having Spotify—to give back to the creators.
I'd also give out upvotes more sparingly overall, since upvoting a post reduces the amount my previously upvoted posts will get paid.
That way you can interact with the site and upvote shitty memes as you normally would, but when it is time to be serious you'll using the paying-upvote instead.
Reddit kinda landes on a similar formula with Reddit gold.