Expression Builder helps me so I don't have to remember the exact names of objects. Or so I don't have to have the names loaded on my clipboard.
Let's say you are in a control source of a textbox and you want to take the value of another field on your form and multiply it by 1.05, to give the amount times 5% tax for the total of a sale. You can type in =[OtherFieldName]*1.05 into the control source box or you can do this:
Click Control Source, enter the Expression Builder, find the name of the control you want to multiply in the middle column. Click it and click Paste, or Double click it. Click on the "*" button (to multiply) then type 1.05 and then click OK, Don't press enter, it puts in a new line that you don't want.
The expression builder allowed you to click on the name you wanted instead of typing it in. This saves some typos that can aggravate you when debugging forms or reports.
There are many things you have available in the Expression Builder: Functions, Constants, objects from anywhere in your MDB. It can be helpful, but it is not necessary.