The general benefit of an HTPC is the broader range of functionality it brings to your television / home theater / music when compared to a simple cable box or provided DVR. One can view movies / TV shows and archive them to disk for re-viewing later on, without the hassle of optical disks (loading, unloading, storage space). The same with music files. Some DVR applications support commercial flagging so one doesn't even need to fast-forward past them.
Further, one is able to accomplish PC functions at the TV like browsing the net, playing games, video chat, etc. I've also integrated several networked security cameras so that I can view them at my televisions.
The networking functionality allows me to store all my media in a central location and stream them to any of several viewing locations in my house.
In all, it is a more feature-rich entertainment experience.
Surely, much if not all of the above could be accomplished with a suitably performing laptop, but that simply isn't the route I've taken. For one, if a person hopes to program a recording for a live TV show, the PC will need to be left on and ready to record the program when it airs. What's the point in using a portable computer if you're just going to leave it in one place to record your live TV shows as they come on? It is possible to use a "backend" server to handle recordings and use a laptop as the "frontend" to view the content, but again, that simply isn't the route I took.