Purpose of static classes in C#

Hello All,

While learning about static classes, i gone through many forums as far my understanding i got to know that static class members can be accessed directly using class name and hence not required to create object of that class which in turn helps save memory and faster execution of program.

So my question is if this is the case then why not always use static class over normal class.

I know my question may be little weird, also i may be wrong with the concepts. Please if any one can explain this in detail with example. would be great help.

Thanks.