Much like Docs, Sheets is the equivalent of Excel and it will come in handy for keeping you on track when writing.

It can be used for various tasks ranging from plotting to character sheets and descriptions, even visual references can be added.

With Sheets you can work out even the crew roster and rotation of a crew of a space ship or the turn-around schedule of the trader caravan in a fantasy story which helps you keep your plot straight and can assist with keeping track of where your characters are at all times

Much like in Docs, you can add comments to specific cells of the worksheet, by just selecting it, right click and writing what is on your mind. (Alternatively you can press Ctrl+Alt+M)

Once you made a comment, the cell will be highlighted and you or another person can reply to your comment and discuss the contents of the cell in question until you get to the point where you get to resolve the issues you might had.