It's not essential, there's much to learn before it. But once you know how to use it, you wonder how you could fly without it before - you simply can't know what you're missing.
I often create new flight plans, change waypoints, create anchors, IPs, with a bit of practice you can use it to set up weather penetration routes and custom glide slopes if you don't have one handy. It can be used to delimit a target area, kill box, no fly zone, SAM range rings (of this one is a lot of work, and can be done much better). It makes you know where you are (if you didn't know anyways, for some reason), and you can probably make it play music (ok I made that up).