I am just as curious as the OP.
Not being familiar with how people use their HTPC systems, I see the following advantages:
1) capturing (recording) stuff from tv
This eliminates need for DVR/PVR/whatever else
2) not having to stream/transfer/burn movies you download and watch on PC
So, if I download something on my PC, I can either watch it there, or stream it/burn it to media, and watch on my home theatre system. HTPC makes this more convenient as I either don't have to stream or don't have to burn discs.
3) Use my home theatre system for gaming and general computing (this assumes you have a home theatre system better than your PC, better speakers, bigger/better tv, etc.)
What are the OTHER benefits?
I've had a home theatre system for a long time (and a PC), and I've never really imagined my life being more convenient by combining them. I like the option of being able to watch TV and do something on the computer simultaneously. Of course you can do the same with HTPC, but it requires a second PC. More importantly, I think, I can't envision a physical setup for gaming, where my keyboard and mouse are where *I* want them, without moving some sort of desk into my HT space. So with a HTPC, I'd have to either ruin my HT space, or constantly move stuff for my keyboard/mouse. Also...cables...ugh. Don't like wireless keyboard/mouse.
If you buy a second PC to be your HTPC, you then get benefits I listed above, without the drawbacks - but then it most likely isn't good enough for gaming, photo/video editing, and so on - so you go back to main PC for that.
So anyway, what are the other benefits? Is there anything I've said that can easily be remedied (keyboard situation, etc.) and I'm just missing it cuz I'm HTPC-ignorant? Please don't take this post the wrong way, I'm not trying to be insulting or anything, I genuinely am curious because I think HTPC is an option I should consider for myself.