You calculate it to make sure it's not a small number. Anything above "small" doesn't matter, but you want to make sure you don't work a year to sell a $19.99 app to a maximum of 500 companies.

Happened to me once... after half a year of development we realized the total grand market for the app, in the form we built it, was 5 (five) companies. Wasn't as stupid as you't think, we found over 100 companies listed when we first did research. Only when we started actually selling we realized how many were big enough, still in business and close enough to actually make use of what we offered. We got 60% of the market, btw.