1) The more opaquely yellow or yellow green snot may indicate its bacterial and you might need antibiotics.

2) You also might be able to tell whether you have a nose bleed or are on the verge of getting a nosebleed.

3) It's also a chance to see if you blew right through the tissue, and if that happens then a search for if it landed on your clothes or furniture may follow

Yes, of course, if you have nose bleed and you didn't look, you wouldn't know. When you go outside with the nose bleed they will think, why are you so disgusting? Unless you want to be disgusting.

It's gross but it's important to know what color snot you are excreting.

http://www.businessinsider.com/snot-color-green-yellow-sinus-infection- cold-2016-1/#-2