Storage

It's not a "why" question. The battery performs important functions. When the battery is dead, those functions are not performed. The functions include maintaining the CMOS setup and the real-time clock.

The battery is dead. Until you replace it, you will continue to have the problems. That's why it has a battery. It needs a battery that has energy. A dead one will not work.

If you owned an iMac or MacBook, you would have to take it in to the shop. They use batteries which are "special" and are not sold to non-techs. Or the batteries are soldered in. How they make money. Lots of it. With PCs, the batteries reside in a holder which is on the motherboard. Extract the battery (looks like a large 5 cent piece) and take it to an electronics store or a dollar store and get one which is the same size and thickness. Unfortunately, there are zillions as it seems each device manufacturer designs batteries which differ by mere millimetres. But being standard battery cells, the voltage is the same. For longevity, I recommend the silver-oxide type which although more expensive, last way longer. Of course, I am old school and maybe there are new types out. That's why taking the cell to the store is important.

the button battery mentioned before is a std non-rechargeable battery which will run for several years. It will run down more quickly if the PC is down for long periods, because it only powers the CMOS memory chip and system clock chip when the PC is without power from the mains. When it goes flat, your CMOS memory can't keep your BIOS settings stored anymore (yours or the default settings). That 's why the PC won't start up as normal. As has been said, change the large (3.0V) button cell for a new one and set your BIOS for default settings to start off. It 'll be fixed!
http://en.wikipedia.org/wiki/CR2032_battery