What's the point of sleeper agents?
It depends on the goals of the country/entity that put them there. Many countries (supposedly even today) have/had what are known as "Fifth Column" agents.
Basically, these are normal people for the most part. They obtain or are given supplies, weapons, etc etc and stash them away. The intention being that when your country is invaded, these 'normal' people wait until the enemy pushes ahead (pretending they are civvies, going about their daily lives), so eventually the border changes and they are in enemy territory. Then they connect with other 5th column agents and start co-ordinating attacks against the invading army, usually alongside partisans or miltias.
Others are established in OTHER countries, to perform similar tasks, obtain weapons, then commit attacks. Supposedly that couple that started shooting up their workplace were 'sleeper agents' who established themselves in the community.
Other sleeper agents may be people who are told to obtain high ranking positions in government or military, so they have to build up a backstory and ensure they appear to be 100% normal to get those positions. Then they establish contact with their handlers and do what they are told, anything from pass information to actively try and disrupt movements or communications.
During WWII, the US government delt with the Caecilian Mafia to fight back in Italy against the fascists. Reportedly, we gave them guns, and ammo, and they gave us maps and helped our troops navigate the country, as well as occasionally fighting themselves.
As the story goes, once we started supporting the Mafia, no dockworker strikes ever happened again for the duration of the war in the US.
In France in WWII, some of the highly venerated partisans who helped fight back using guerrilla warfare could be considered 'sleeper agents' though very loosely.
https://en.wikipedia.org/wiki/Fifth_column
https://en.wikipedia.org/wiki/French_Resistance
https://en.wikipedia.org/wiki/Collaborations_between_the_United_States_government_and_Italian_Mafia