It's a bit like asking why you need an editor to read your manuscript or why you need a typesetter to typeset your book (instead of just saving a Word document in pdf and sending it to the printer).

Take a look at Smashing Magazine, for example. That's a static html website built with Hugo. Guess who built it? Professional frontend developers :-)

Maybe you don't need a professional for your site, or maybe you do. If you are a company that is sold on the idea of having a very fast, elegant, interactive, and yet searchable site, then maybe you do. Also, if you are a frontend developer and already have the skills, then maybe you'll like this tech for your own site (I know many frontend devs do).