Other web developers and potential employers will be able to look at your work and know that you are a person who likes to keep up with changes in technology and make sure that your knowledge and skills are always current. It will make you look like a real web professional.
Use web standards combined with best practices for accessibility and give your clients a chance to talk about how they cater to all people, and how they find it important that everybody can use their services or find information about their products. You will also avoid the bad publicity that can be caused by shutting out visitors like disabled people, Mac users, and mobile phone users.
Well-structured markup that separates structure and content from presentation is generally much more compact than table-and-spacer-image-based tag soup. Documents will be smaller and faster for visitors to download. Like it or not, there are still many, many people connecting to the Internet through dialup.
If your site has a hosting plan with a limit on free bandwidth usage, smaller documents will reduce costs - provided traffic doesn’t increase.
Would you rather wade through many kilobytes of multiply nested tables and spacer images or just browse through a clean and well-structured document when you need to update your site?
Removing, inserting or editing presentation-free content is much easier and more efficient than having to make sure you get all the presentational cruft right. Using CSS to control layout also makes it much easier to make site-wide design changes.
There is no way anyone can guarantee with 100% certainty that the documents created and stored electronically today will be readable in a hundred years. Or even fifty years. But if you separate content from presentation and use current web standards, you have done the best you can to ensure that your content can still be read even after you’re gone.