Combat as a mechanical resource
If you are using XP progression (which is the default way to play, according to the PHB) and you want to give your player characters some catching-up, then you could use an easy combat encounter for this purpose. Similarly, you could give the party items as a reward from such encounters.
Combat as a pacing tool
While D&D is essentially a combat engine, not all campaigns are heavy on combat. Often, story-driven or intrigue campaigns tend to be light on combat. They may essentially involve the party going from location A to location B. If the players seem to get bored by this pace, the DM could add the occasional easy combat encounter to maintain player engagement.
For example, if the party is traveling by ship and the DM wants the voyage to seem more substantial than a scene transition, then an encounter with mutineering pirates or ambushing sahuagins could make the voyage more interesting.
Combat as storytelling
When you narrate your game world, the DM's medium isn't limited to the words they say when the player characters walk into the dungeon and look around. Instead of communicating via exposition, the DM can use interactive in-game challenges to convey the same message.
Much like how environmental exploration can be a means of storytelling, you can use combat encounters as a form of narration, worldbuilding, and foreshadowing. For example, the DM may communicate that a crypt is infested with undead via hostile undead, rather than exposition. Or, suppose the PCs are exploring a forest with a big bad CR 3 Winter Wolf at the end, and the DM wants to foreshadow "Here be wolves." Instead of exposition via NPC dialogue or narration, they could instead send some CR 1/4 Wolves as a greeting party.
Disclaimer: Adding extra combats is not universally beneficial to every game, especially when time is limited. Eschewing combat in favor of a battle summary ("You stab the goblin and get 100 XP and 5 gp") or narration ("The sign says there are zombies inside") is more efficient than a time-consuming battle, but potentially less engaging. However, for the reasons given above, these extra easy combats can be beneficial to some games, and the DM should consider the tradeoffs before adding them to their campaign.