I've been told to set up my website using wordpress but I'm not sure about this.

Although wordpress is much easier than setting up a website normally, should you use it? I'm going to be designing websites and coding them for people, but if they are using a website that is not based on wordpress what would I do? Should I get used to designing the website without wordpress and with wordpress? I've been told that if you use wordpress you don't have to deal with errors, but is that a good thing? If I am dealing with errors in the coding on other peoples websites then if I only have experience with wordpress what am I going to do?

I was wondering, do you have to have a content management system for your website to work? Or can you just write the pages and then upload them to your server?