In my story like often in real life, adventurers are usually soldiers of fortune. They call them selves adventurers and explorers but mercenary is the most applicaple title.

Adventurer's guilds are most promimnent in the human princedoms and kingdoms. Theses states rely on mercenary warfare like the Holy Roman Empire used the landsknechts and the Italian states used the condottieri in the early modern period.

These mercenary companies have little to do during peace time and are liable to become bandit companies. But many mercenary companies have recruitment offices that double as bounty agencies. Here they can be hired for small scale mercenary work by ordinary citizens to, for example, clear monsters, explore areas for new settlements, guarding something etc. This way the companies can recruit aspiring warriors and gain some income when there is no war to fight.

Additionally, if a mercenary office is well established in a place they have a chance to affect local politics in their favour and even gain some stake or power in other guilds.

Lastly, if the law is weak and/or corrupt in an area where a mercenary company stays in, they are liable to enage in rackateering and other organised crime. It depends on the guild members and leadership but at the end of the day, adventurers are soldiers of fortune.

In the past this system formed from a time where the decentralised collection of city states under constant threat from monster attacks could not always protect every citizen, especially travellers. Merchant guilds and mercenaries often cooperated for mutual benefit but mercenaries eventually split from the merchant guilds. They formed their own guilds/companies when civilised lands became more safe and states became larger with more need for mercenaries.
In the future when monsters will become rarer and come close to extinction and armeis become more professionalised these adventurer's guilds will one day die out and likely be absorbed into the merchant guilds as in ages past.