There are times when there literally is nothing better to do:
1. Building the NP in a tundra forest gives you many specs instantly and if you aren't in caste you have nothing better to do.
2. Sushi pushes your pop up before you have decent slots in filler cities. Normally these are cities where I'm trying hard to get into the number 1 or 2 pop position for UN purposes. For instance a one tile city (say in the gap between to BFCs on a diagonal) may have 8
and 8
from corps. Say I'm in rep due to SSE and eman for happiness/cottages. I will handily outgrow the spec slots I need until I get a courthouse, lib, temple, and a forge.
3. You can't run whip and need hammers ASAP. This is very rare, but if you are locked in eman or serfdom (only ever for diplomatic reasons) and you need get a border pop or shave a turn off a wonder.
4. Food balancing. Particularly if I've skimped on pottery (i.e. block settle with oracle shot to CoL) I sometimes find myself with 1 more
than I need for stagnation and optimal output from mines. Rather than go unhappy and promptly lose the new citizen I will toggle a 2
tile to a citizen to start eating down the surplus. Either I'm waiting for a new happy resource to come online before growing or I'm waiting until I can whip the unhappy guy away in a 3 pr 4 pop whip for something big.
Really though, citizens are specs of desperation; running virtually anything else is superior. If you have many of them, you should be considering caste.