Sometimes, writing a bit of detail in a document can be really helpful. It helps not only the writer in structuring their thoughts, but also offers a place for subsequent team members to go if they need to know why a system is designed the way it is, rather than just how. Adding some detail of the intent of the design can be very valuable when it comes to changing that part of the system in the future.
Intent is an important thing, and all too often it is not recorded. Sometimes the discussion about whether code comments are important misses this very thing. A good comment can convey intent - what someone meant the code to do, rather than what it actually does. If those differ then that can be the bug that someone is looking for.
A pre-created design document template can also act as a guide for people to follow. You need to provide reminders for the sort of security questions that need to have been considered, so having a template with the sections already in it gives people a checklist of the aspects they need to cover.
Teams can make a lot of progress with a word processor and a template. However, as you progress, you might want to migrate to a tool that helps with very fast reuse of patterns and standard practices. This is where a custom design tool can help.