Are all of those functions necessary? If yes, why?

Are they all "necessary"? Maybe not. I mean... the code works without declaring individual functions for each aspect.

One of the big reasons for doing it though, is readability. For something like this project, it's relatively simple... but say I have an application that needs to do a host of things. I need to pull data from two different database sources, then parse it and format it all to be appended to an HTML page. Something like this could easily become very, very long, and thus, difficult to read and follow. Abstracting some of it away into seperate functions that are named well could very well help in understanding what it is you, or someone else looking at your code are reading. Particularly useful if you're coming back to your code after a long period of time, when it's not particularly fresh in your memory.

Another reason is reusability. Again, in something like this, the app is relatively simple, so it might not be all that important to do this. But I think something the lesson is trying to get across is that it's a best practice because as your code gets more in depth, and complex, you may find yourself needing to do a similar task, tens, hundreds, maybe even thousands of times. Personally, I'd rather write one line, then scroll through thousands of lines of code to find the place where i need to copy from to then scroll back and paste. Or worse, have to rewrite a complex chunk of code I've already written.

Hope this helps.