OK! Let's get this out of the way right up front. I am a copywriter, ergo I am biased.
Fine, now that the disclaimer is out of the way, let me tell you the 11 real-world reasons for you to stop writing your own copy or trusting it to your "web person". I like web designers; I work with them and hire them. But they are techies, not copywriters. The general rule of thumb (with apologies to that rare breed with both talents in equal abundance) is that "Techies can't write and Copywriters can't tech."
So, if you want your marketing results to soar, find an experienced, professional copywriter who: