What is the purpose of Mark as a Company on Mac Contacts?
Two reasons:

Firstly, it's so the contact displays the company name when you receive a call instead of a personal name. 

Secondly, it also sorts it in alphabetical order into your contacts by company name and not by a personal name. This is especially useful as contacts are sorted alphabetically by surname and not by first name, ie. Tim Cook is listed under "C" in your contacts but if you were to have "Apple Computer" you would want this sorted in the A's and not the C's.