I'd be very leery of using one of my limited warlock spells known on Catnap until high level. Catnap can only work on a particular character once per long rest, so it's not like the warlock can do it on themself to regain slots more than once per day. Maybe at 11th, when you have 3 slots so you could cast 2 and have one for this once per day, and you have a good number of spells known. But in the single digits of level I'd stay away.
"You make a calming gesture, and up to
three willing creatures of your choice that you can see within range"
...er… you can use it on your self and you get two spell slots starting at level 2 but this is a 3rd level spell so even if warlocks could get it they would already have 2 slots. Since warlocks reset spell slots on a short rest you would use your second slot to get both back... the only lose their is a limit of spells known. BECAUSE you only have 2 slots until level 11 getting that other slot back before a fight is more important not less. because it means your trading 1 known spell for the chance to recover your second spell slot when you might not have time to rest other wise.
Also, better for the same reason:
- Magic initiate (warlock) - first level spell hex - hex counts as a warlock known spell can be case again with pact slots per sage advice.
- Pearl of power , Ring of Spell Storing* , and Rod of the pact keeper
So imagine a ring of spell storing + catnap prior to a fight. You have 20 mins, you cast a spell into your ring, you cast catnap and restore your spell slots and start the battle with 3 spells instead of 2. Or you fill the ring the nigh before, have a fight and use all 3 slots, take an hour and recharge 2 cast one in the ring then catnap to get back to 3 so you can have 3 spells for 2 fights that are close... still niche because if you can get 2 hours the usefulness of catnap drops to almost nothing... but then what does any other class use if for? It's still a "we have 20 mins gather your strength and take some hit dice because they are not far behind!"
Edit: It does get better at level 11 and again at level 17 but again the problem here is that the only class that any of this matters for is a class that doesn't get the spell... so short a 5 level multi-class dip your not getting it, which then means no 17 levels of warlock.