Did you get left out of playing shiny quests?
If you're using EUv1, you must use EUv2 in order to go online with your characters after you convert them to Ver.2. The first time you try to go online with any character, PSO will ask you to input both your Ver.1 and your Ver.2 serials, and if the serials you enter do not match the region of the disc you are using, PSO won't let you use those serials (and thus that character won't be able to go online).
This is a one-time problem, though, so once your character has been online at least once with EUv2 so that the game accepts your serials, you can use JPv2, USv2, or EUv2 with that character from that point forward. EUv2 prices went up a lot when it was the only Ver.2 disc capable of going online, but now that BlueCrab disabled the Hunter's License check in JPv1, JPv2, and USv2, those discs are useful again.
You'll need a GD-ROM to do the offline quests, Challenge mode, and to safely play in unlocked teams, as a CD-R copy will FSOD if you try to load character data when you're not on Pioneer 2. You can avoid freezing by playing quests or in locked teams, but not everyone likes to do that. If you're using BlueCrab's loader to patch the game, you could use an EUv2 CD-R to convert your Ver.1 character to Ver.2 and go online for the first time, and then you could buy a cheap USv2 GD-ROM as your main disc. They're cheaper, and BlueCrab's loader will defeat the region lockout and disable the HL check for you, anyway, which largely eliminates EUv2's advantages.
The main benefit of EUv2 now is for people with a PAL Dreamcast and a BBA, as EUv2 is the only Ver.2 that can go online without needing any boot discs; however, doing that means you won't get the map fix that BlueCrab's loader provides, so you won't be able to play Famitsu -Maximum Attack- v2 in Ultimate.