This will prevent germs on your hands from entering thean organism and may harm you.
This is essential because it is the only way to avoid pathogens from the intestinal tract or urine being passed on to other people - not only when shaking hands, but also when touching a door handle, for example.
Covering your hand when coughing or sneezing is better than sneezing or coughing freely. But the Viruses then stick to your hands. That's why you should always use a new tissue when sneezing. Alternatively, you can sneeze or cough into the crook of your arm, so your hands stay clean.
Source: https://www.youtube.com/watch?v=jR52yNp_bWE
Little researchers