My question is what do the engine and Apu bleeds actually do cos I use them without understanding their purpose.

It is air that is bled off the compressor section of a jet engine prior to the hot section. Depending on the aircraft, it can be used for many different things. Here are some typical uses:

• Pressurization
• Engine starts
• Ice protection
• Environmental control (cabin heating)

Aircraft Bleed Air Systems

Aircraft Bleed Air Systems

On the ground at the gate/on the ramp, APU Bleed will provide air for heating/cooling/and engine start on airplanes that have pneumatic start. Since you're flying a sim and not suffering in the elements, Just start the APU sometime before start and that should give you air for starting engines.

Once started, Engine bleed will provide the pneumatics that your specific plane requires. This gets to be airplane specific. Business jets might have pneumatic start or electric start. Large transport airplanes use it for Enviro, Anti-icing, and engine start. 

If you get into a certain classic Boeing...Battery switch on, IRSs to NAV, and start the APU. This will get functioning since you need IRSs to align and they take time. APU bc you are freezing are sweating inside. Make it a habit.

Contributing to the discution, at the begining, it may sound unfamiliar but big jet engines use pressurized air to start up. The air is ducted to a device on the engine that turns air pressure into movement of the aircraft engine itself. After a given rpm the engine ignites itself and becomes self sufficient. This pressurized air may came from APU, aircraft engines or an External Power Unit.

Once APU or aircraft engines (aircraft engines  are also source of pressurized air) are available on their rated rpm, they can feed other systems which rely on pressurized air like anti-icing, aircraft pressurization and climatization.