1. So you don't repeat yourself
You may have come across the DRY, for don't repeat yourself. If you find you need to write the same block of code twice, it would be better to put it in a function. That way you've only written it once.
You might say "I'm not writing it twice, I'm copying and pasting it", but that's just semantics. Also, if you change something in that code, you have to remember to change it in both places. Maybe it's something you can easily find and replace, but maybe it's not.
It's all about cutting down the amount of work you have to do. And the number of places you might have errors.
2. To make it readable
Let's say your script has 400 lines of code. You don't look at it for a year, but when you come back you have to make a change. Good luck finding it! You might have to read through all the lines and figure out what they're doing. Maybe you've written good comments explaining what each bit does. But you could have replaced some of those comments with functions.
If you're reading through your code and you have this:
getHighScore(); displayHighScore(); playGame(); calculateHighScore(); setNewHighScore();
you can guess what's going on in each of those functions without having to read all the lines within them. If each of those functions contained 10 lines of code, you've just saved yourself from having to read 45 lines of code.
In conclusion: write functions.