A very, very good business guy can call up his rolodex and sell your software X times for 6/7 figures a pop prior to a single line of code being written. To do this, you need a) a rolodex, b) personal credibility, and c) sales skills. That guy is richly worth the half of the company he's getting in a two-man founder scenario.

Sadly, really good business guys are rare like really good engineers, and totally incompetent business guys are (bare minimum) as prevalent as programmers who can't FizzBuzz. There are many people who can, by writing their best idea ever on a napkin, deprive the world of a perfectly serviceable napkin.

(Really good business guys are priced higher than really good engineers, principally because their skillset necessarily includes not being totally stupid about money, negotiation, leverage, or scarcity.)