IMO, the purpose is to have fun - just like with any other game.
VG has some pretty good lore, excellent character classes, some very nice content, a massive world that feels open and huge, and fun loot. The diplomacy minigame is pretty cool too, even if it doesn't quite feel like a (typical) MMORPG element.
SOE is working hard to improved VG, and it's coming along nicely, IMO, but this was a game that was forced to release before it was done, that had a very rough release, that's had all kinds of performance issues (you want to have a clean high end PC to run it well), and had all kinds of bugs and technical issues. The game is touted as '3rd gen' or 'next gen' but the graphics and eye candy (especially considering the horsepower required) aren't quite there and sometimes the game itself is missing features you'd find in other popular MMORPGs. Considering the designers, the 50m budget, and the 5 years, you have to wonder how they spent that time and money. The design is/was ambitious - and some of that design is in place and impressive, but some elements are equally lacking.
Overall, for me, the positives of the game outweigh the negatives, and the game is improving with every patch. The best part is that the character classes are all very well done and fun to develop, and the worst part is the technical problems, like random CTDs (which are decreasing with patches).
I didn't start with VG at release though - since it looked like more of a trainwreck than EQ2 was at launch - and when I did start VG I knew there were gonna be some technical issues. I've been surprised though, as the horror stories I'd read and expected haven't really affected me, and overall my time in VG has been very fun.
VG was basically created by the same people that created EverQuest, and it often shows, but VG is definitely beyond old EQ and more like other modern successful MMORPGs. VG has the soloability of a WoW or EQ2, and the quests of a WoW/EQ2/LOTRO, while still maintaining some of the old school EQLive feel that some of the new MMORPGs (like LOTRO) are mostly lacking.