Why is it important for a plant to make a hormone that keeps its seeds dormant.

This topic is especially interesting to me. Did you know there are trees that release some of their seeds only during fires? And by the way, the seeds are only “activated” if they are exposed to fire as well.
This is important for regeneration if an area is destructed (secondary succession: destruction after a fire or tsunami or other natural disaster) These seeds that are dormant create one the first organisms to appear after destruction, and begin the recreation of a healthy environment. So dormant seeds are essential to life after disaster.
The example I gave above supports this in a way. Plants want their seeds to germinate (develop) in the best place suitable for that specific kind of plant, whether it be water, land, or after fire. So the seeds will remain dormant, or asleep, until they reach that suitable environment, which has features that will trigger the seed into germinating. Think of an animal during winter. They remain dormant until the environment is suitable for them to come out and live. If they come out at the wrong time, they will freeze, starve, and die. The same goes for the seeds.
Hope this helped a little :)