A1 notation is familiar because it names a column with letters and a row with a number. R1C1 expresses both coordinates numerically, which can be easier to generate or reason about in automation.
Direct references
A cell such as AA27 can be described as row 27, column 27. Direct conversion is mechanical when both coordinates are absolute.
Relative R1C1 references need a base cell
Bracketed references such as R[-1]C[2] describe an offset from the formula cell. They cannot be converted correctly without knowing that starting position.
Why developers use R1C1
When code generates repeated formulas, numeric offsets can be easier to calculate than letter-based column labels. For manual workbook work, A1 remains easier for most people to read.