If you want to sell your website or just know the value of your forum, then getting a website appraisal from others would be useful. You would often be bias with your own site because you will most likely overprice it but when you ask others for website appraisal, then you can see how much people might be willing to pay for your site.
I recommend not using any online appraisal tools because those are often very wrong about estimating the sites worth, either too low or too high. They're only a script so only so much they can do or take into account.