Why should I join a guild?

The greater bonus is on the elements, guilds can boost elemental armor and damage by a percentage defined by how much gold members have donated to the guild, the more active the guild members the greater the bonus.

The bonus apply to minions too so is generally a good idea to join a guild.

You will be able to unlock different individual rewards. Each reward is dependent on the amount of contribution you are currently at. Rewards can be as high as tiered fusion boosters. While players can get individual rewards, Guild leaders can unlock bonuses for the entire Guild to take advantage of which includes things like +10% for each element in attack and armor for your minions, attack for weapons, and defense for armor.

Your Stronghold will also soon be getting a new layout and new traps in order to keep other pesky players out of your gold stash. This also includes the ability to get new, stronger minions to guard said gold. This update will also bring 11 new armors and 11 new weapons, all starting at Tier 3 and can be upgraded to Tier 5. And, of course, there is new content in the form of three new single player missions to work your way through.

More info can be found here