why does the create-post.tsx component need the 'use client' directive in the default project?

I'm a n00b with create-t3 but have used all these technologies in the past (except tRPC). Recently, I've been more focused on DevOps so it's my return trip to the Javascript ecosystem.
I follow Theo's YouTube channel and wasn't sure if it was something he'd mentioned on a video or something I saw in the docs, but the create-post.tsx file in the default ct3-app uses the 'use client' directive and I can't seem to find the explanation anywhere. I know I either saw or heard it and it's driving me nuts.
I'm familiar with difference between client components and server components in React (I was a React dev before I started getting more interested in cloud/devOps).
Forms would be a great use case for server components and I'm just curious why these were pre-written like this? I'm guessing it's something with tRPC? I'm going to start playing with it so will update if I can figure anything out, but if anyone knows the video or the place in the docs I'm talking about, I'd really appreciate it!