Hello and welcome to Imperial Hero.
First of all, there are other people you can consider allies. You will help them and they will help you in times of need and you will all face your difficulties together.
There are also some other, practical, benefits.
First of all, every guild owns a headquarters. There, the leaders can build various buildings that give bonus to crafting, gathering in that province and a guild stash. Furthermore, for every guild level the leaders can build an equal number of outposts in dangerous or safe provinces. These outposts can have some buildings too that offer additional bonuses - another guild stash, some bonuses to heroes' attributes when the guild members fight in the province and such.
A guild can also fight for domination in no-mans-lands. By challenging and defeating the owning guild, your guild can claim one of the limited number of outposts there or even the castle itself.
I think pretty much this is it. If I missed anything, people here will make sure to add it.
P.S.: When you open a topic, please make sure to post it on the right section and refrain from reposting in more than once. I've deleted your topic in "Guilds" category for this reason. Have a nice stay.