This is why rooks are worth 5 and bishops are worth 3, despite both having apparently equal freedom of movement (just rotated 45 degrees). The bishop will be able to access strictly fewer squares than the rook even in the best case, and usually much fewer. And the bishop is stuck on one color.
They denote the power of pieces in relation to eachother, as a general rule with lots of exceptions, and reflects many material imbalances that are often comparable in strength(usually in the endgame). The unit of measurement here is the material value of a single pawn.
3(3x1) pawns are often enough compensation for for a knight(3) or a bishop(3 + ε).
3 minor pieces(3×3) are often sufficient compensation for a queen(9).
One minor piece and 2 pawns(3+2) are often sufficient for a rook(5).
Two rooks(2x5) are typically slightly stronger than a queen(9).
Two minor pieces(2x3) are typically slightly stronger than a rook(5).
There are different systems that are derived from number of squares attacked. One thing to note about these systems is that pawns are not created equal. Rook pawns attack one squares less than other pawns, for instance.
Such evaluations are only a rule of thumb intended for beginners.
It's all context dependent. There are positions where a Bishop is more powerful than a Rook. Other things being equal, Bishops tend to be preferred over Knights in serious play (especially when you've got a Bishop pair, which is synergically more powerful than one Bishop times two), even though both are theoretically "worth" 3 pawns. Two minor pieces are usually worth more than a Rook and a pawn, and more often than not it isn't a good idea to exchange the former for the latter, even though the total value appears the same (3+3 = 5+1). And so on.