The best way to become a good writer is to write a lot, and keeping a diary can really help you to accomplish that. The good thing about diary-keeping is that you can write anything you want about something you know very well: your day. You can write about your day in general, or you can focus on specific events.
You can write about something you learned, or you can just write out your thoughts, especially when you’re feeling sad and you don’t want to tell anyone in particular. It’s flexible–and that flexibility will help to keep all of your writing ‘arms’ in practice. Different types of writing call for different techniques.
By keeping a diary, you can practice writing in a variety of formats and styles, which will prove useful when you need to write more “serious” pieces.