The register keyword helps a user to declare the register variables, and these were actually supposed to be faster as compared to the local variables. But the modern compilers these days are very commendable at the optimization of codes. Thus, the chances of making a program faster just by using a register variable are very rare these days.
Unless and until we are working on any embedded systems where we are aware of the process of code optimization for any given application, the register variables are of no use.