The starter battery stores the voltage generated by the generator while driving in order to maintain the on-board power supply when the engine is at a standstill. This is necessary in several respects, for example to ensure that alarm systems continue to work, the clock in the cockpit doesn't stop and the radio doesn't 'forget' its favorite station. These tasks account for the majority of a starter battery's work - and not, for example, the actual starting process, which is then usually followed by a longer charging phase during driving. While a voltage of extreme strength is fed to the starter for a relatively short period during the starting process, the battery delivers a voltage of moderate strength to the vehicle electrical system over a relatively long period in standby mode.
Longer idle times, frequent short trips and the self-discharge that already occurs put a strain on the battery throughout the year, not just in the winter half-year.