What do background apps / services actually do?

[AC Question]

Among my installed apps is Instagram, not an app with a particularly heavy load I'd have thought but I notice it has a background service running constantly on my phone, what does it do? It can't be down to handling notifications as I have other apps where I receive notifications just fine without there being a background service. Facebook messenger is another one I can go a day without even opening it but it will drain around 5% just sat there doing whatever it does.

What do they do and why are they needed?

 

[Crashdamage]

Most services and processes are not actively running, they are just cached in memory. Let me explain a little more.

They may be inactive, literally doing nothing, just parked in memory for quick retrieval should they be needed again. As such they use no power and have no effect on the device's performance.

They may be service-only processes. Again, they are parked in memory for quick retrieval, but sort of have one eye open, listening for a command to get to work again. These use a very small amount of power. Usually it's not enough to be concerned about. The keyboard is a good example of a service process. Unless you are typing, the keyboard stays cached in memory, only very minimally awake, waiting for a tap on the screen to call it back to action.

Then there's active processes. These are actually running, using CPU cycles and power. Facebook is a notorious example of an app that leaves processes running. However, usually an app does so because it's necessary for it to do its job.