1. It's more professional, so it makes you look more professional. Simply put, if you want to be successful, messy website examples that don't hold their structure in every browser are not the way to go. Anyone who visits a website you design should see exactly what you want them to see.
2. Professionalism makes your clients look good. Their site visitors won't complain, so they won't complain. Less headaches for you.
3. Maximize traffic- The goal of most websites is repeat traffic, which isn't going to happen if you lose all the people still using IE6 or any version of Netscape.
4. Reduction in load times - Well-written code means faster loading. Dialup is still out there in rural areas, and like it or not, a poor economy may herald its resurgence. It's also worth noting that load times may factor in SEO protocols in the near future.
5. Better search engine ranking - SERPs love clean, well structured code.
6. Using web standard code helps ensure accessibility program compatibility for people with disabilities.
7. Updating source code – Fat, messy code is a nightmare to wade through for updates, especially if it wasn't your code to begin with. Web standard code is easier to understand and repair, for you and for the guy who might someday take your place. If you've ever inherited a morass of nested tables within tables, sourcing multiple CSS stylesheets in different folders, you already know why web standards are important.
8. It's makes good business sense. Using web standards makes coding easier for everyone. It's easier to share and collaborate is everyone is on the same page, and it's a great deal easier to debug. That means faster markup and less time spent debugging and updating.
The bottom line is that using web standards is less time-consuming, which translates to lower development costs and less web developer frustration, for now and for the future. It also means increased traffic, and for the developer that usually means glowing recommendations from clients, resulting in a better professional reputation and more business as a result.