It isn't always possible to be near an outlet to keep a charger running. A laptop is supposed to be portable. But a user may still need to be on their computer more than just 2 or 3 hours their batteries allow them. That can turn into a real problem if the person can't keep their computer running because lithium ion battery stocks its battery just died.
Deterioration is a function of temperature and charge level. Don't make the mistake of storing the battery fully charged, as that is the state where it breaks the fastest due to heat. Charge it to take it out and put it like that on the back seat of your car avoid direct sunlight and preferably put the mini lithium facts notebook on a firm flat surface while operating it.
Laptop computer uses: If you keep the battery in your laptop for everyday use, and power it from the wall socket, unplug the power every few days and let it run off the battery, as a battery at 40% capacity lasts longer than one at 100% capacity. if you don't travel with it, you should remove the laptop battery when on fixed power.
Why lithium bettery stock is this important? If you have a relatively new PC the only problem you will get are error messages relating to the clock on start up. No big deal. If your PC is older then a low battery could be a nightmare. I had a PC which once every few months refused to boot. Rather than take the PC in for repair or start replacing parts myself the first thing I did was look at the BIOS settings. It was soon obvious that the BIOS settings were corrupt (wrong bus speed). I put a volt meter on the battery and sure enough it was reading 2.2 volts.
Briefly relating back to overall size... many lithium 18650's will differ ever so slightly in length (measured in millimeters). This is mainly due to an added protection circuit. The circuit helps to prevent the battery from over-discharging, since Li-ion cells can easily become damaged if their power is drained below a particular voltage. The protection circuit also prevents the cell from being over-CHARGED as well. These cells are very sensitive. They must be handled carefully, gently, and as described above, must never run-down too far, or be charged over and above their threshold. They can actually explode if severely mistreated!
Using these easy tips to keep the battery in the best condition won't do wonders, but will let you use it to the maximum number of recharges. Ever wondered why some laptops keep their maximum capacity for 3-4 years, while others dive under an hour of runtime in one? This is why.