Item hearts are a useful way of tracking interest. If something has tons of views but no hearts, it tells me that while the first image is compelling, there's something about the listing that is a turn-off. If something has lots of hearts but hasn't sold after a while, it tells me that people like it, but it may be priced too high.
Shop hearts, on the other hand, are mostly an ego-stroke. People heart shops because they like the person as often (or more often) than because they like the products or they multiple products in the shop.
Also, the more hearts you have the more often you are going to come up on the 'suggested shops' features.
For me personally, I heart items as a bookmark. Their items I may want to purchase at a later date or I might want to feature them in a treasury or my blog.