You pay that subscription cost each month and have unlimited game time with whatever server you choose.
You might create 5 different characters and have each of them on different servers (good if you have friends scattered all over the place).
In Free To Play MMO's, paying a subscription usually gets you bonus stuff like extra character slots or in-game items.
There's a huge list of benefits to paying a subscription... the game is always being patched, updated, new things, special events and constant server maintenance. I've always found it worth it... although I've never liked that games like WoW charge for expansions, I think that's gouging, particularly when games like EVE who could use the money more don't charge for their updates.