Among the people I know still using Last.FM, it's somewhere between having statistics about your music, and a recommendation engine. It isn't about playing the music, you can do that elsewhere. But by having data on every single piece of music you've listened to, it can recommend music you will like, and potentially recommend people with shared musical tastes as well. There's a feature to compare your musical compatibility with others.

For me, many years ago Last.FM recommended this weird electronic band that I'd never heard of, with the strange name "Boards Of Canada". That Last.FM recommendation was responsible for introducing me to my 2nd most listened to band of all time (just behind NIN). 2026 is many hexagons, dandelions and an inferno later.