In my post, Addressable Market:  Not Market Size, I make the point that an addressable market is the sum of all revenue generating opportunities available to a company based up on its model. 

This means that the addressable market for a company may not only be different from the current market size, but also different from that of its competitors.  For example, imagine that there are two dating websites - one that generates revenue from advertising and another that charges its members a fee.  If the ad-supported site can generate $10 per user per month and the other site charges a $20 subscription, it makes sense that these two addressable markets may not be the same (even if you assume that they both appeal to exactly the same addressable population).

The fact that an addressable market can be unique to each company in the space, highlights the importance of providing this calculation to investors, as it will help inform them about the opportunity specifically associated with your company.