say you write a nifty program for remote access in your home directory and name it rm for short then exec it with the flag r for ron and * to remote access for all. Problem is that /usr/bin is in the front of your path so the shell will pick up the remove every file you see command instead. By explicitly requiring you to invoke the current directory for a file it indicates that you are aware of where you are in the heirarchy.
regards,
daemon