It is never* worth it to spend an action on healing
D&D characters are as effective at 1hp as they are at 100hp. If it is worthwhile to cast healing spells (expending spell slots and actions) on someone who still has HP left during combat, then it would be even better to spend actions on other things (attacks, cantrips) that bring the combat to an end sooner or mitigate enemy damage, and then spend the spell slots on healing after combat ends, when actions are abundant.
But there was that asterisk...
There are a few situations where the above doesn't apply.
Sometimes, your teammates are at 0 HP
A character with 0 HP is not dealing damage, but is a single Healing Word away from being able to deal damage again. In this situation, healing is a no-brainer. There is almost nothing else you can do with that bonus action (plus spell for the turn) that would be more effective than getting your downed teammate back in the fight.
Technically, this is just a bonus action and not a real action, but it means that you can't spend your action for the turn casting other spells (aside from Cantrips), so it could be argued to still be spending an action on healing.
Sometimes, your teammates aren't as effective at low HP
I, as a player, choose to play suboptimally when my character is at low HP. Sometimes I have my character panic as they realize that death is one misstep away. Sometimes they get angry to drown out the fear. Sometimes they accept death and make themselves a target to try to go out in a blaze of glory.
Sometimes, characters are selfish and will yell at the cleric for ignoring them when they're in pain (though I haven't played such a character yet).
If your party acts like a Darkest Dungeon expedition, then these sorts of roleplay concerns can make it worthwhile to cast healing spells, even during combat, even when it is not necessary mechanically.
Sometimes, dropping prone is a big deal
A character who falls to 0 HP falls prone and has to stand up before they can attack without disadvantage (or avoid being attacked with advantage). Spending an action to heal someone before they hit 0hp avoids having them fall prone. Depending on initiative order, failing to do so might the enemy might be able to take advantage of their prone condition to send them right back to making death saves.
Sometimes, the specter of the instant death rules looms
There is a (somewhat under-discussed) rule in 5e where if an attack reduces you to negative one times your maximum HP, you die instantly. At low levels (before you have access to Revivify) or against monsters which destroy the body on kill (like Gibbering Mouthers), a single lucky enemy crit can take a character from low HP to gone forever.
Casting Healing Word on a downed ally creates the perfect conditions for this to happen: they are at extremely low health, can be attacked with advantage, and are still a dangerous combatant from the enemy's perspective. To avoid such situations, it can be useful to keep your teammates with a comfortable buffer in hit points.
Finally, house rules may apply
In the popular computer adaptation of 5e, Baldur's Gate 3, characters which have just been healed from 0 HP cannot take actions on their next turn. Almost needless to say, if your table plays by that or similar house rules, any strategic or balance wisdom that relies on the rules as written will need to be reconsidered for the new context.