Orbits are not reserved. Orbits are on a first-come-first-serve basis. Orbits are managed, though.
Space debris is a major problem. Sure, space is huge but it is quite populated with debris. Famously, a speck of paint bore a hole in the ISS (not all the way through.) This problem is known as the Kessler Syndrome, and it is a huge problem.
Space debris is tracked by NASA and the Department of Defense. Tracking space debris is a huge must. Being blind to space debris leaves the ISS susceptible to "pecking."
Agencies announce launches to let people know that the rocket that the agency is launching does not have a warhead and let other agencies be aware of the satellite. No agency is going to launch a satellite in the ISS's orbit (disregarding rendezvous.) No-one wants to lose a one-billion satellite nor four astronauts.
Orbits are not reserved per se, but they are managed to avoid collisions. Agencies spend a lot of time tracking spacecraft. Without this managing, we would have lost quite a few satellites. Agencies sometimes fail to track all of them, resulting in collisions. One such example is when a French reconnaissance satellite smashed into Ariane rocket debris. You can see more collisions here.