There are a few reasons why apps might be running in the background. They could be providing you with information, such as the weather, or might be doing things like checking for updates, and syncing data. They could also be waiting for you to give them a task, like adding an event to your calendar. Or, they might be running a process that doesn't need your attention, like backing up your photos. In any case, if you're not using an app, it's best to put it to sleep so it doesn't drain your battery.