In my 3 year old's case, it's largely a matter of re-enacting scenes from her life or things she has seen in cartoons. For example, she has three rubber duckies in the bath, one of which is larger than the other two; that's "Mommy duck" and sometimes she tells the "baby ducks" that she has to go off to work because she's the concertmaster of the orchestra, at which point the baby ducks complain and ask her to stay home with them instead.
Young kids struggle to think about things in abstract; re-enacting situations is a natural part of how they come to understand their experiences. In this case, it's the experience of being left alone with Daddy because Mommy is going to work.
Adults run through things in their heads, sometimes repeatedly, sometimes obsessively for much of their lives (in the case of trauma). We just feel too limited to act it out with toys - but why not? We'll even watch movies, plays, read books, partly because they enact things that trouble us - we'll do anything but be seen to act it out ourselves.
Young kids struggle to think about things in abstract; re-enacting situations is a natural part of how they come to understand their experiences. In this case, it's the experience of being left alone with Daddy because Mommy is going to work.