The ESC key has many uses, but we will stick to how it is helpful in QuickBooks here. There are two main reasons I use it that I can think of off-hand.
First, there are the times that you click in to a transaction (putting you into Edit mode) either on purpose or by accident, and want to be absolutely certain you do not inadvertently make any changes to that transaction. So, instead of clicking on 'Save and New' or 'Save and Close' buttons, just push Escape to return to the previous screen you had open without recording any changes.
Second, there are the times that you have 'Zoomed' in multiple times from a report to follow the history and info of a number (for instance report to transaction to transaction history to another transaction, etc), and would just like to return to the report you began with, without having to close a bunch of windows first or inadvertently changing any data. Just use the Escape key to take you back the way you came, closing one window at a time, until you are back at the beginning.
I'm sure once you are used to reaching for this 'Tool', you will wonder how you ever survived without it, and will begin to find some of the other uses it has.