Well, I'm an office worker and eat both breakfast and lunch at my desk. I always put my food and drink on my little mug rug - it keeps everything neat and tidy, protects both my desk and my food from the germs ON my desk! At home I use them just because they are pretty and I like pretty things around me.
I guess more "practical" folks might think they're just useless, like knick-knacks. To each their own, I say!