Overlay is an easy setup and the emulate settings are fun to customize.

I've been using StreamElements for about 3 years now, mainly because of the customizable overlays for Twitch livestreams. While other services have some sort of pre-made overlays with little to none customization, StreamElements let me customize them in many ways. I've made a few short videos to use when people follow me, subscribe for me or tip me and it works perfectly! Also, the possibility of realtime updates to the number of followers and the username of the last follower, among other stuff, is an important point for me. Last but not least, it's very easy to apply the overlays to any streaming software, there's even a plugin to integrate it with OBS and make it easier, but it's optional (which is a good thing).

variety of items that help make lives more dynamic