I use it for:

* Reminders

* Checklists

* Idea collation

* Fleshing out and prioritizing personal tasks

* Reference notes - e.g. passport number, tax numbers, frequent flier numbers, anything else I might want to input into a form one day.

* Calendar

* As a place to draft emails/letters/documents.

There's so much crossover between all of these things that it helps to keep them all together.