you can buy banners for your guild at 2gold each dont think you need them straight away and from CB they play some role in RvR but we can look into this later.
from what i gather about banners is that one guildie is the standard bearer carrying it into combat, banner has abilities depending on trophies attached and guild rank.
whilst being carried bearer cannot take part in combat, however it can be planted in the ground freeing up the bearer but making it an easier target for the enemy.