If you subscribe you become a VIP member and get 500 points each month and a special PvP mode among other, more superficial perks. The freeloading scum must play the game to scrounge together points in 5-20 point increments (for example by completing quests, exploring areas, killing certain monsters, a number of things, really). You can also pay good money for these points. You don't really need points to play the game, but they certainly make life easier.
Points can get you, among other things :
two(?) more bag slots
a shared chest for your account
the content from expansions
additional content like highlevel quests
boosts and crafting materials/recipes
a reset on your daily quest completions (there's a limit)
other stuff I don't remember anymore, it's been a while.