I'm a DIY type of guy, and I go back far enough that I can write HTML without use of an editor....

And I still pay a designer for our website. Why?

1) We now have to have the website optimized for mobile devices, as more traffic comes in that way than conventional computer...
2) Browsers - OMG! I thought it was bad in the old days. Today I can't get Microsoft products to work right with some major commercial websites let alone what you write. A designer will have more structured tools to make sure what you're doing will work with legacy platforms as well as new tools.
3) ISALGIATT It Seemed Like a Good Idea at the Time. There's 101 ways to write a front-end editor for HTML, and they come and go - even the 'good ones' like Front Page from no less than Microsoft are extinct. Even if your web page is accessible, if the maintenance tools used to write it are sunk, you are as well. Make sure your designer isn't enamored with something that can't be fixed with other tools.

The 'test with grandma' is so true, particularly for ticketing and purchasing. The problem remains that a lot of websites back-door to a shopping cart ap that is truly second-rate and you can't fix it, or have much defense if security goes nuts and they accidentally release stuff. YOUR computer might not have secured customer information on it, but what about theirs? It's a helpless feeling to realize a shopping cart interface is really, for all intents, the worst part of your experience and you're stuck with it. Your designer has a lot more experience with these tools than you will.