If you have a web site then you must learn, at the minimum, how to make simple changes and upload the changes. (There is one exception to this).
Even if your site has been designed by a professional there is always going to be a need for some change. Do you want to pay for, and wait for, a designer to do a simple alteration which could take you minutes ?
So you will need to learn the basics of HTML. You can pick up a good understanding by studying the code of existing sites and there are numerous resources on the Net.
You may find however some of this too technical, ideally you want to learn from the angle of setting up a business site.