“Custom” web design implies that the customer is the “designer”. It’s very important to know, that it takes a lot of professional knowledge to be able to design a website. As creative as the customer is/can be, not always they know the rules and guidelines involved in the design. The customers know their business the best and they can bring their wants and needs, so the designer will be able to provide them with the best designed website/art for them.

Some clients run into a website they like and start fantasize about how they will design their website, and instruct the webmaster to deliver them with their vision as they have it build in their mind. Without considering any of the design methods or website development compliance it may need. 
A website designed wrongly would often end up hurting the customer rather than do them good. The customers are best at what they do with their business and choosing professionals to drive their business further…
Webmaster/designer/developer/engineer are talented people whom acquired tools and knowledged over several years. When a it comes to design/develop a website, it takes more than just an “eye” for design. A professional web designer/developer/engineer will be able to serve any customer with their vision in the right professional way which will elevate their business. 

The website is a crucial tool for your business, if you respect it and the professional team behind, you will see the results better and faster than you have thought.
It is very important listen to the professional people who are about to design/build/develop your website, if you really know want you want and they can see your vision clearly, everyone will be happy.