If you use the prototype for attaching methods then you have one copy of the method shared between all the objects that use it. If you don't use prototype then each object will have its own separate copy of the method.
1. A separate copy for each uses more memory.
2. If you have code to change the method then having one copy is easier to change than having an unknown number of separate copies.
Helping others to solve their computer problem at http://www.felgall.com/