What is it for? Does it have to do with alignment? I have searched Google and have no luck.
I'm just going to throw the kb - select all cells on a worksheet official reference here as an answer.
It's also on this kb- Select cells ranges rows or columns on a worksheet -
That button selects all the cells of the spreadsheet. :)
It highlights all cells of your sheet. It can be used if you want to set every cell to a certain font style or something like that.