Catnap is a shorter short rest

If a target remain unconscious for [ten minutes], that target gets the benefit of a short rest …

This is supposed to be a helpful spell not an offensive one. You use it on allies to give them the benefit of a short rest (which normally takes at least an hour) in only 10 minutes.

Short rests have many useful effects like refreshing some class features, allowing hit dice to be spent to restore HP, and ending some harmful effects. This spell would allow allies to reap these benefits in at least 1/6 the time.
Examples where this might be handy

This is obviously going to be the most useful when time is of the essence. Maybe you are being pursued by enemies and you need to refresh but don’t want them to gain too much ground. Maybe you are the ones chasing an enemy and time is important for the same reason. Maybe you have to be in a place at a certain time and you don’t have long much time to get there.
Other situational uses

I suppose the spell could also be very situationally useful if you needed a way to gently knock a willing elf unconscious. It works for any willing creature of course, but elves are more limited since they cannot be affected by sleep spells which is a common way to impose the unconscious condition in a non-aggressive way. I actually had one time where this came in handy in a campaign of mine, but I really don’t expect this to be a very common use at all.

You could also, very theoretically, use this on an enemy that trusts you and willingly submits to the spell. Maybe you go undercover and disguise yourself. This seems pretty unlikely and I’m not sure that Catnap would be the best spell even to use in a case like this and it would depend on how your DM defines “willing”, but it could be another use.

Rapidly taking a short rest is the primary and intended use of the spell.