So you wish to know what in the world would valid HTML do for you? How is it a benefit to know how to do so? Well, that question actually isn't a bad question but has a simple answer that can be explained. It comes down to expanding your knowledge of HTML. Many people who have gotten involved in web site development have probably built sites without knowing if it is valid. It is normal for that to happen. How would you know this? If you think about it, everyone goes through it when involved with web site building. I'm going to go over a few reasons why having a valid HTML web site will benefit you greatly than a non-valid one.
Complete Knowledge: Knowing the codes is the simple part; the next step is trying to put them together in the right order. It means having end tags when necessary and not forgetting any. It is not hard to learn HTML if you wish to go in depth but going into complete validation.
Future Work: Believe it or not companies will look into what you can do as far as being a web master goes. If they can see that you validate your pages and make sure there is no formatting in order issues, then you will be most likely to be hired than those that don't. On the outside, the pages may not look like a mess on the inside of the coding it could be a disaster.
Browser Compatibility: While maybe not a huge issue, it should however be taken into account because one browser may view the page perfectly but another might leave it looking like a mess. I have done that in my earlier stages with messing up on the coding issues. A very common example would be between the two major browsers in Firefox and Internet Explorer as it could be one perfect and the other a bit messed up. It is rare but it can happen.
Evolving: You know HTML is going to evolve and keep growing in technology so it would be best to keep up with it. If you are not up to speed when it comes to working with validating web pages, you will be lost and the coding may not show up properly on an updated browser.
You will really need it if you have to go into the next stage called XHTML. When you work with that language, you will need it to be valid. They do have to be lowercased but that is another story for another time. Overall the main reason to have validated web pages has to do with showing everyone that could be interested in people with building web pages that you can complete the job and have it look the way it needs to.