Pathfinder: Kingmaker (the videogame) is based on the Kingmaker Adventure Path published by Paizo for their Pathfinder roleplaying game.
Now, Pathfinder is heavily based on D&D 3rd edition (3e) and especially on the 3.5e revision of the ruleset, for that ruleset came with an Open Gaming License that allowed third party publishers to legally build on top of their system.
When 3.5e publishers moved to D&D 4e, which had a way stricter license, Paizo wrote their own ruleset, trying to win the 3.5e audience.
In 3.5e, cantrips (and their clerical counterpart, orisons) are just level 0 spells. These spells usually don't scale in damage or effect (only their chance to have an effect raises with the general competence of the character, which means increased to-hit and save difficulty checks that the opponent has to save against) and apart from not receiving extra 0-level spell slots because of an high ability score and a rule that states that these actually count as level 1/2 spells when determining how many pages they take on a spellbook or the cost of acquiring a scroll, they are effectively spells like any other.
What really changed in Pathfinder, making Cantrips and Orisons a different affair (and eventually having the cure minor wounds orison removed from the game), is that 3.5 Wizards, sorceres and other spellcasters had very powerful spells but supposedly very few of them, so that they couldn't keep spamming spells all day long, needing to revert to more mundane ways to deal damage and solve problems. Pathfinder authors decided that the cantrips were not that powerful and instead of having wizards switch to crossbows, they gave them infinite cantrips per day, so that they could keep being magical all day long. (Crossbows are still better than straight damage spells, but some other spells are really useful, like being able to spam previously useless spells like "+1 to the next skill check" or "light" or "create water").
Anyway, the idea that you leveled up but still had access to low level spells is familiar to the players of all three editions. Usually those spell slots get filled with utility spells, backup spells in case you end up spending all higher slots on a taxing day or spells that don't care about save DCs and keep being effective throughout the game despite their low level.