*The short answer is:* you don't, but it saves about *1ms* of CPU time (on modern CPUs).


*The longer answer is:* 
Exec replaces the process image of the current process with the process image of the executable you exec.
That means that the moment you exec, the shell process that does the `exec`ing gets completely destroyed and replaced by the `exec`ed program.
When you don't `exec`, the shell forks itself, execs in the fork, and waits around for the child process to exit, collecting its return status, in the hope there might be additional commands to run afterwards (`fork` + `exec` is the standard procedure by which new commands get spawned).
Since there are none, the `fork` is a complete waste of time and you might as well exec directly and save on that `fork`ing time. 

It's essentially a microoptimization based on the knowledge of how process get spawned on Unices.