The Fishing Guild in Runescape is a nifty little area for paying members to fish. One must have a fishing level of at least 68 (which may be reached through potions too) to enter the guild. When you ask if the fishing guild makes you catch fish faster, the answer to that is no. You still catch fish at the same rate you would anywhere else, however overall fish caught after an hour is greater at the fishing guild due to its close proximity to the bank. Also, the fishing level 99 skillcape is bought inside the fishing guild.
You ask what is the point of the guild. What are its perks? Well, firstly is the one I just mentioned. All the fishing locations are of very close proximity to a bank which increases the total amount of fish you can earn. The quicker you deposit into the bank the quicker you can go back to fishing.
Another perk is that the fishing guild offers a great variety of fish in very close proximity. Some of the fish available to catch in the fishing guild include shark, swordfish, tuna, bass, and many others that I did not list. For this great variety people can fish different fish either because of their level or their target fish.
Finally, although this is more of a social perk rather than an in-game bonus, the fishing guild is a guild known to be quite populated. This means someone at the fishing guild has opportunities to chat, trade, and meet other people pretty much on any realm at any time. Obviously some realms and times will be more populated than others, but it's not like some other guilds that are notorious for being empty all the time.