Text boxes are useful because you can easily place them anywhere on your spreadsheet. You can link the contents of a text box to a cell.
First draw a text box on your sheet. Then when the text box is selected click in the Formula bar and type = then click on the cell to link to. You can link to a cell in other sheets. Changing the cell contents will change the text box contents.
This is useful to put dynamic messages around your spreadsheet. You can use a formula in the cell to create a dynamic text message which will then appear in the text box.