Guilds offer advantages to players that you can't get anywhere else.
They offer small bonuses but every 5 guild levels they will increase giving more and maxing out at guild level 25.
The Possilble perks
Slayer - 2% damage per level - 10% max
Mercurial - 2% speed per level - 10% max
Blacksmith - 4% armor per level - 20% max
Toughness - 2% hp per level - 10% max
Energetic - 2.5% stamina/mana per level - 13% max
Whetstone - 2% bleed per level - 10% max
Diseased - 2% poison per level - 10% max
Skullcrack - 1.5% stun per level - 8% max
Martial arts- 2% Melee resistance per level- 10% max
Take cover- 2% Ranged resistance per level - 10% max
Skepticism - 2% magic resistance per level - 10% max
Death-cult - 2% health-stealing per level - 10% max
Note - Energentic and skullcrack are decimals and round up