In a recent post we were asked how to change Excel’s columns back from numbers to letters so that the formula references refer to A1 instead of R[1]C[1]. You see how to do it on the ‘Change Excel columns back to letters‘ post. But why does Excel change to R1C1 format in the first place?
So the obvious way is that you, or someone else working on your computer, changed the setting as shown in the other post.
More likely is that you opened up a spreadsheet (probably from another person) that was set to R1C1 style. Once it is opened, Excel assumes that you want that as a default, so from then on, all spreadsheets you open (even if they were A1 format) will convert to R1C1.
This is similar to the issue with manual and automatic calculation methods. If you open a spreadsheet that is set to manual calculation method, then from then on, all spreadsheets opened will start as a manual calculation until you change it back to automatic calculations.
In the R1C1 case there is nothing to worry about. It is just an Excel format and the spreadsheet is the same and works the same in either A1 or R1 C1 format.