The viewfinder in a mirrorless is a full-on digital display, meaning it can be overlaid with all sorts of useful information that simply isn't possible on a DSLR's optical viewfinder. You also see exactly what the sensor is actually capturing. A lot of people (me included) consider mirrorless to be the future, whereas the writing is on the wall for DSLRs. DSLRs exist because sensors and displays didn't use to be good enough, necessitating the optical path, but they are now.

Also, mirrorless is significantly smaller than DSLRs for the same sensor size. Smaller and lighter is always welcome.