When you are making a big financial model, where assumptions are in different worksheet and you want to see the effect of change in your profit by changing assumptions in different worksheet, ‘watch window’ comes in handy.
It essentially saves you from the hassle of going to different worksheets time and again.
From F1 : The Watch Window makes it convenient to inspect, audit, or confirm formula calculations and results in large worksheets
Go to Formula -> watch window -> add watch and select the cell/s you want to keep watch on
Watch window appears as separate box, you can embed it above the formula bar by going near the top of box, press right click( plus sign appear), drag it above the formula bar.
If you think you left with little space to see excel, adjust the size of box or double click on the head of any tab, like if you double click on home tab (where home is written), the ribbon will go giving you space to see, reverse action will bring back the ribbon