Joining a guild means you will attain guild bonuses. This means you'll get more credits and experience performing certain tasks, i.e. combat, passenger hauling and cargo hauling.
Next to these 'automatic' benefits a guild can help out in many ways. They can get you free or cheap resources, items and services, sponsor certain investments, provide valuable information on the guild forums & many other things.
Of course guilds operate in diverse ways. Some guilds are tight communities based on friendship while others are more economic in nature. This is why you need to look closely at the character of a certain guild before you join it; choose one which suits you best.