It's for downloading the officially released roms from the HTC site when they come out. If I remember, HTC's site requires you to enter your s/n just to see the entire list of downloads for a given phone. The reason for asking for each country is that the phones are released in each of these markets with a different set of serials, which (for language, legal, and carrier reasons) prevents customers from downloading a ROM that's not intended for their phone.
The reason for asking/posting on here and having a full list is that people can:
a) keep an eye out for new roms without relying on the owners from that particular country to check regularly
b) make it easy for owners of a phone with a different country's serial to get the new rom, which can then be flashed on if they have HardSPL. Some people do this because they want to have the latest features while still using official roms (not sure what they have against cooked roms, but each to their own.)
Phones: Nexus 4, HTC One, Blackberry Z10, iPhone 4
Tablets: Nexus 7, Samsung Galaxy Tab 10.1 7510, Blackberry Playbook
HTC Inspire 4g (Desire HD/Ace), HTC Raphael (AT&T Fuze),HTC Polaris (Touch Cruise),HTC Cingular 2125 (Tornado), Dell Axim x3, HP Jornada 548