Summoning demons might be more useful than conjuring animals depending on the circumstances and desired results. For instance...
Demons are immune to poison
One trait that all demons have in common is that they are immune to poison damage and the poisoned condition. If you are fighting against enemies which rely on poison damage and status effects, they will have a harder time against a bunch of demons than against a bunch of mundane beasts.
Demons are inherently scarier and do weird things
For the common person, hostile wild animals are certainly scary, but hostile demons are undoubtedly more so; and where animals will simply physically attack their foes, demons tend to have extra magical or unusual abilities which make them more versatile. If your goal is to create as much fear and chaos as possible, a squad of demons will do that job more effectively than a gaggle of beasts, even if, according to game statistics, they're actually equivalently dangerous. Where town guards might be confident enough to take on a boar or bear, they might flee in terror at the sight of a demon... even lesser ones.
It also provides the DM a convenient character option for villains to use, since summoning literal demons is a good way to paint an antagonist as obviously evil.