Adding comments at the sheet level in planning software facilitates better collaboration and communication among team members. This functionality allows users to add, view, and respond to remarks directly in the same sheet where the data or report lies. This differs from a cell-level comment, which is more granular.
Scenario: A global financial institution uses planning software for financial reporting and performance tracking. A sales leader makes significant changes to a revenue model during the forecasting process. In an Excel environment, he would communicate his changes in email - which would readily get buried in everyone's inbox.
Solution: By enabling sheet-level comments, the sales leader can enter in his notes for the entire sheet. Those notes are available to everyone with access, saving them from digging through emails. When reports are pulled on the revenue forecast, the notes are readily available.
The best systems will offer both sheet-level and cell-level comments with all the same features - file uploads, comment thread format, notifications and so on.
Also consider comments on an entire row or column instead of the entire sheet. Those can be useful when breaking back a value for an entire account and noting the change for the entire row, instead of a summary for the sheet. Sometimes summary notes are hard to pinpoint when looking at large sheet.