An advantage of being able to serve ads in about 210.000 individual apps, is that pretty much every audience is reachable. Why? Because for every interest, every hobby and every daily situation; an app exists. Your or your client wants to serve ads in apps for horses, sailing, running, music, soccer, tennis or whales (just some examples)? It can be done.
Advertisers/clients with a very specific audience, most likely are able to reach their target group. The more adexchanges a programmatic advertising platform is connected to, the bigger the chances for actually finding, reaching and engaging with their specific customer-base.