Wildfire,
Having caves is not mandatory at all,however it is highly reccomended. some types of cichlid are "rock fish" and as the name suggests,they live among the rocks. As with any fish , its beneficial to try and create as close to its natural environment as is possible if you want your fish to flourish.
Also certain cichlids are more aggressive than others,and caves or rock formation provide a degree of shelter for any smaller fish,or for the female if she's being harassed by the male.Without this some fish stress out more and are likely to die of increased stress levels.....unless they are bullied to death first.
(i introduce a large male golden zebra to my tank,and noticed my male auratus took an instant dislike to him. when i checked 2 hrs later the new fish was dead)
Even in a 20 gal you can have rocks. If your fish are small i would put in a clster of small rocks. Or if your fish are larger i would have three or four larger ones,perhaps creating a bridge for them to swim through.
Having certain types of rock in the tank can also be an effective way of altering your ph if needed.
Craw.