Design Notes, as with Dreamweaver.
To help to keep track of which layers/objects are used as 'controls'. e.g. I am currently designing a new logo. One particular layer that controls the outer borders needs to change from 2px stroke to 3px stroke depending on whether the background is black or white.
Also, just having notes to explain what 'is' and why 'it' is together with what it is actually for i.e. its' function is extremely helpful and a must for technical files.
It's obviously better to be able to refresh my memory with the appropriate notes instead of having to meditate for half an hour before I get back to work.