HTML is the language of the web. What could you do with it other than building websites? Quite a lot, actually. HTML is not a programming language. It does not have to be compiled and it does not create an executable file. A HTML file is just a text document with special tags which tell a browser what to with it's contents. The browser is the program that does all the hard work. Think of it as a Word document and the MS-Word Program. HTML can be used to create files that look and feel like a website, but contain project reports, technical presentations or even your resume.
The tasks mentioned above can be done with Word or PowerPoint. Why should you go to the trouble of learning to code in HTML to do something that can be done very easily with a little knowledge of the keyboard in Word? The answer is you don't need to - Unless you are particularly interested in making spectacular documents which are extremely portable.
The advantages of using HTML lie in it's format. Being a plain text file, it cannot unintentionally carry a virus (I believe it is possible to write extremely malicious java scripts. This is something you have to want to do, it cannot creep in on it's own).
You can jazz it up with images that tile the way you want them to, colours to highlight the important bits and animation to draw attention away from the price tag and small print.
The file sizes are incredibly small - My resume is 40KB in Word format, but the better looking HTML version is just 4KB. Think of the time you save e-mailing attachments. Still need convincing? You don't need to send an attachment. The code can be sent as the mail and a HTML document will be created when the recepient opens the mail.
Pure HTML works across platforms and across browsers. As long as the user has a browser, your message will get through. Not being a program, it cannot crash the user's system (though a buggy browser may challenge this statement. See 'browsers' for more details).
HTML was intended to aid the exchange of information, using tags such as 'Acronym'. This gives you a pop-up box containing information about the tagged text. For example, moving the mouse over HTML gives you the expansion. This is very useful when creating a document containing many abbreviations, acronyms and technical terms as you don't need to clutter the doc with this information. It will retain it's crisp layout while continuing to provide all necessary information to the user. HTML has a lot of potential, most of which is wasted by the vast majority of sites out there.
HTML is also useful for personal use. One of my first projects was an address book using frames. Click on the persons name in the left pane, and you get all the info in the right pane. Each person can have unlimited details since it is not a database - this is very useful when a friend has 3 e-mail id's, pager and mobile, office phone and one at home as well where you're not supposed to call after 10 'cos his wife disapproves. Try designing a database for that! All right, you can, but it won't be practical. My address book occupies less than a hundred kilobytes, and can be viewed on just about any computer.
With this we come to the end of the technical section. I have tried to give you a glimpse of the wonders that can flow from your fingers. The satisfaction you feel while creating something, seeing an idea take shape and form, watching it change as it grows, realising it is a part of you, is unmatched by any other pleasure this world has to offer. Now go build your website. Class dismissed.