Maha, one day it will be us coming to you with questions; your quest for understanding SolidWorks is admirable!
This is a very good question. If the cam shaft plane is in the center of the cam shaft and the cam plane is in the center of the cam then using a coincident mate on the two planes produces the same result. However, SolidWorks must do much more calculating to create the width mate (calculate the center between the ends of the cam shaft, calculate the center between the two side faces of the cam and then through whatever "magic" SolidWorks uses under the hood, makes the two centers align). With the coincident mate all SolidWorks need do is align the two planes (much less calculating). This is why the coincident mate is considered a faster mate and would be preferred over the width mate for this situation.
I hope this answers your question.
Okay now I understood. If both parts origin planes are not at centre then width mate is useful. If you take care when sketching the parts then you can avoid using width mate.