If you remove the battery on the motherboard then the computer wouldn't have any way to tell the time.
This is also the case with mobile phones. If you let a phone discharge and then not recharge it for more than a few weeks it will also "forget the time" because the small auxiliary battery is discharged completely and nothing is powering on the real-time clock.
You could try to power on an old mobile phone if you have one and check it yourself to see that it "forgot the time".
This is how the battery looks. https://yandex.com/images/touch/search?text=mobo%20battery&source=tabbar
I had to buy one a few times when mine started to last less then a day. I had to configure the clock everytime i turned on the PC.