Everything that needs electrical power in the car depends on the battery.
The battery takes care of the power management and serves as storage for the electricity reserves in your car.
You need electricity to start, but also, for example, to keep the lights on when the engine is off. The alternator ensures that the battery is charged when the engine is running.
Over time, the battery retains less and less of its capacity. Batteries become far less efficient in cold weather which can result in your car not starting.
During most eVHC or health checks performed each time you visit your garage your battery will be tested. The results can be found in your Car Passport.