Let's say you're writing an application in PHP. That application needs to connect to MySQL. Did you create PHP? Did you create MySQL? Did you create the lib

Let's say you're writing an application in PHP. That application needs to connect to MySQL. Did you create PHP? Did you create MySQL? Did you create the library that enables PHP to talk to MySQL? Do you create the system libraries each of those needs to operate? Did you create the operating system those run on? Use what's available. Be efficient. If you can use WordPress and it suites your clients needs, use WordPress. That said, WordPress is pretty unwieldy if you need to do heavy customization, and using something like a framework might be a better option. I no longer make websites for money as there's not enough money in it for me personally, but I've built sites on WordPress and from scratch previously, clients don't really care as long as it works.