Well, pubic hair is basically like eyebrows and eyelashes for your vagina. 
Like the eye, the vaginal canal is a sensitive part of the body open to the outside world!
It is susceptible to invasion and infection by bacteria, fungi, viruses and protozoans. 
As a moist environment with mucosa-lined membranes, it can provide the exact niche preferred by some of these foreign invaders. 

The coarseness and shape of the hair makes it easier to trap foreign particles like dust, pollen, pathogenic bacteria....


So if you're like me, you might be thinking, "Ok, but is that still necessary in the modern era? How likely is it that foreign particles are going to get into my Lululemon leggings that I can barely get into myself?"