ITX PC Build
The main reason to bother with an ITX build is not because I specifically need a small system, but to encourage an economical build. Limited board and case space tends to force me to economize on things like "32 gigs of ram" and "four hard disks" which aren't strictly necessary up front (or, on the storage front, with TECT, at all.)
After buying those parts, I could pair with existing laptop optical drive and hard disk, and then later upgrade to a better boot SSD and/or add a good discrete GPU. (Something GTX760/770-class.)
"Regular" PC Build
The main reason to go with a "Regular" PC build is to allow the expansion I had originally planned on, as well as aspects of the motherboard/system such as 32 gigs of ram and/or advanced connectors. In this build, I can also split RAM installation up over time, etc, to keep initial cost low.