To communicate...to sell online...to build awareness...to do some good even.
It could be all of the above or something completely different. But here's the thing. If you don't know why you or your organization doesn't know why the website is there, then you need to rethink. Take it down and spend the money and time on something else...
At the last organization I worked for, this was a pretty obvious problem. With a few exceptions, the owners of the various web properties, didn't really know what their online presence was achieving, or even more worryingly what they wanted it to achieve...
What this resulted in was a confused series of messages that our users had to negotiate before they could engage with the services and information offered. To be fair they aren't alone and are now expending serious resources into transforming the website into something more useful.
The lesson is to always be clear about what it is you want to achieve and to be focussed and relentless in ensuring that your online channel aids the process of achieving this.
Not easy, but essential.