The art of deorbiting is about cleaning up after yourself.
A satellite that has reached the end of its lifespan will effectively stay in its orbit for many many years unless acted upon by another force. This will become space garbage that can pose a threat to future missions or other satellites.
So it may be wise doing a controlled deorbit manoeuvre to have the satellite or vehicle burn up in the atmosphere once it's no longer of any use.
The same goes for payload delivering rockets - once it has delivered its payload it will just end up as space garbage - also here a controlled deorbit is adviceable.
Satellites in geo orbits are so far away that a deorbit burn will require a massive amount of dV making it close to impossible. In such cases it's normal to send the satellite to a graveyard orbit instead. Usually a few 100km above the geo orbit, where there's no real orbit advantages or satellite traffic.
- A worn out satellite in its last days. Using the remaining onboard propellant to perform a controlled deorbit is advisable at this point.
- Successful deorbit burn. The satellite has reduced its speed using its remaining propellant, causing it to drop towards the atmosphere, eventually making it burn up in the atmosphere.
- Payload delivering vehicle now dead in orbit.
- Using ground based conjunction scans we can get an overview of potential hazards. Some are our own leftovers, others are from other actors.
For example a country thinking it would be a great idea to fire a missile at one of their old satellites causing a massive cloud of space debris and permanently increasing the risk for all space operations.
Ensuring your satellites and vehicles has enough propellant remaining to do controlled de-orbits or graveyard burns is key to success - and looking after everyone else!