Some people don't have very powerful PCs.
Full HD can be decoded on the graphics chip, such a card don't cost more then 30€, a CPU that can decode Full HD don't cost much.
fast internet speed and/or a DVD burner
DSL 2000 is probably sufficient to stream 720p, it downloads a HD scene depending on size in 30-90 minutes and it's possible to watch while downloading using VLC since Windows allows to access a file from different processes even if the file is currently written to. DSL 2000 is about 25€ per month in Germany and it's pretty much the minimum you can get in Germany, it's good enough for pretty much anything, most people have much faster DSL 16000, I have only DSL 2000 and can't complain.
A burner costs 30€ but it's so much easier to store everything on a hard drive using a HTPC or some device like WD TV Live.
Others (like me) just don't like watching porn (or anything) on their computer.
No problem with a HTPC or some device like WD TV Live.
And putting a DVD together yourself via download & burning is a bit of a chore and also a somewhat soulless experience.
Optical media like CD/DVD/Blu-ray has many disadvantages, I always try to use something else instead like a hard drive or flash.
It's much better to have a big collection of scenes accessible through a media center interface then having a big shelf with DVDs which takes much room and everybody can see it unless you hide it and then still somebody might find it, a hard drive is discrete. Dealing with the discs and it's menus and chapters is rather cumbersome compared to PCs and a media center interface. A HTPC takes some expertise but some device like WD TV Live should be manageable even for less tech-savvy people and it's also affordable, about 100€.
http://www.wdc.com/en/products/products.asp?driveid=735http://www.youtube.com/watch?v=0ahAkLPlH0U