I raise mine for meat but don’t have enough broody hens to hatch enough chicks to put enough meat on the table.  My flock size and freezer space is limited so I have to hatch throughout the year to keep meat on the table.  I also play with genetics a bit so I need to hatch my own eggs.  While I prefer a broody hen top do the job, an incubator gives me control of when and how many I hatch.  I only hatch as many as we will eat.
There is no way to tell you how many out of 25 will hatch.  There are so many factors involving heredity, health, nutrition, and fertility of the parent flock, how and how long you handle the eggs between when they are laid and when you start incubating, then a huge number of factors that can effect hatch during incubation that the number can be all over the place.  The professionals that may hatch 1,000,000 chicks a week will probably get about a 90% hatch rate of the eggs that go in the incubator.  Some individuals claim a better rate, a lot of us get worse.  I’ve had anything from a 30% rate to a 100% rate.  On average I probably hatch around 80% of the eggs that go in the incubator but that can vary a lot from one batch to another.  Broody hens normally do better than we do but even with them I’ve had some really awful hatches and some excellent hatches.