Cantrips fill the role of second rate damage for casters on turns where they concentrate on spells. It is typically less than the damage possible from most melee or ranged weapon builds since in most cases they do not add the casting stat (eldritch blast + agonizing blast being a notable exception).
Cantrips also provide some utility abilities (like Prestidigitation, mage hand, druidcraft, minor illusion but these are probably not the ones you are referring to).
If you run campaigns with 2-4 encounters/long rest, casters can afford to use spell slots on many combat rounds and combat cantrips are much less essential. In a longer day with 6-8 encounters, the spell slots get spread more thinly and the damage cantrips give the casters the ability to contribute a little bit of scalable damage to the encounter. Keep in mind that spells can be hit or miss, sometimes no one saves, sometimes everyone saves. Casting Banish or hypnotic pattern might do nothing or completely change an encounter. Cantrips also give casters something to do when spell slots are expended.
Cantrips also provide some utility abilities (like Prestidigitation, mage hand, druidcraft, minor illusion but these are probably not the ones you are referring to).
If you run campaigns with 2-4 encounters/long rest, casters can afford to use spell slots on many combat rounds and combat cantrips are much less essential. In a longer day with 6-8 encounters, the spell slots get spread more thinly and the damage cantrips give the casters the ability to contribute a little bit of scalable damage to the encounter. Keep in mind that spells can be hit or miss, sometimes no one saves, sometimes everyone saves. Casting Banish or hypnotic pattern might do nothing or completely change an encounter. Cantrips also give casters something to do when spell slots are expended.