Here's an explanation I found.
"The cold cranking ampere (CCA) rating refers to the number of amperes a battery can support for 30 seconds at a temperature of 0°F until the battery voltage drops to 1.20 volts per cell, or 7.20 volts for a 12V battery. Therefore, a 12V battery that carries a rating of 600 CCA tells us that the battery will provide 600 amperes for 30 seconds at 0°F before the voltage falls to 7.20V."
The starter motor use a lot of amperes when you crank the engine and you need a battery that can supply all the power the starter motor want. The standard battery is fine when it is fresh, fully loaded and warm outside (25C/77F). The battery will degrade with age and use and it will loose some power (in layman's terms it won't last as long) and you will notice this very fast when you crank the engine. This effect will also be noticed when it is cold outside, the colder it gets the less the power gets. At 10C/50F you have lost roughly 15-20% of the power making the engine crank slower and that can be enough to not get the engine to start. This is why you can have the bike (or car) start just fine in the warm garage but it won't start later out on the cold street.
As I understand it the Sprint engine has to reach a certain RPM before the ignition starts to give a spark. It seems like the standard battery is very close to it's limit in the best conditions and when it gets older or it's cold outside there's just not enough power left. With a battery with high CCA you will have a bigger margin of usable power when you crank the engine and even when the battery gets older or it is cold outside you will stay above the limit and the engine will start.