Aggregated routes are used to summarize multiple routes into a single entry. So you can minimize the number of advertisements sent out and decrease routing table size.
You have three /24 networks you need to advertise. You can define a /22 aggregated route and advertise that instead.
As long as at least one of the /24 networks is active then the aggregated route will be advertised.
Generated routes are very similar to aggregated. But they use the concept of a "forwarding next hop." This is the next hop that is associated with the primary contributing route (the active route in the routing table that is longest match.)
Aggregated routes are very common when using BGP as you have push out a lot less information. Generated routes are not as common (in the commercial space that I work in.) Generated routes are often used in conjunction with routing policy to push specific default routes when certain conditions are met as defined in the policy.