You get to use everything you learned in movie business. You talk to actors, if you do it right - and I haven't always done it right - you should be shaping the material all the time. And the other thing is, you get all the blame when you direct and it doesn't work. You get slammed. So that's another reason to know what you're making, why you're making it, and make it the way you want.
Acting in particular is a fun job when you have a good script. I don't know about acting when you don't have a great script. I'm gonna say that's not a great job, it's kind of a dumb job. But when you have a good part in a good script, it's the best job, in a way.
In acting, you're only responsible for your part. When you're directing, the load is on your shoulders, so you have to feel a strong connection to the material. It's a higher bar to get over.