Guilds are just for the medals which you can get 5*s with (360 medals, more available 5*s in the future. Theres also materials and tanuki but not as good as the senki). Not really much point other than that as well as the fact you can get to know more people who play the game. You can also farm gold if other guilds get 0 parties and surrender (20k each time you attack when they are surrendered).