Besides providing a group of like-minded people with whom you can play the game the guild also provides access to
A personal bank that provides additional storage for your gear.
A shared guild bank that can provide access to free gear.
Vendors that sell items that are not available anywhere outside the stronghold.
Additional game content (heroics, and tasks in the stronghold) as well as quests
When you turn in stronghold missions you get XP and shards or vouchers. When you turn in the shards and vouchers to the coffer you earn Guild Marks. Guild Marks can be spent at the Guild Vendors for Guild Only equipment.
How guilds work
The buildings around the stronghold have various functions. Producing material for the coffer and providing boons are the two main functions. Some structures help defend the stronghold against raids. The higher level a structure is the greater the quantity of the benefit that it provides. You can think of a guild as a major campaign.
How you can help
Your guild needs your help to grow. You get out of it what you put into it. The obvious immediate benefit is, the more you donate the more Guild Marks you earn. But, by contributing to the current build goal you help your guild to improve its structures which will provide greater benefits to all guild members.