Mostly to see the usage patterns in your particular site. Wikipedia or other sources might tell you that "approximately X% of the devices that use e-commerce s

Mostly to see the usage patterns in your particular site. Wikipedia or other sources might tell you that "approximately X% of the devices that use e-commerce sites are mobile phones" which would be useful in figuring out that you should have a responsive site, but analytics could tell you "this one element doesn't display properly on mobile devices and therefore 95% of the users on mobile devices don't use this particular functionality".