Deterministically signing a Tx with RFC6979 returns v, r, s, where r and s are the 2 values used in standard ECDSA signatures. v = 27 + (y % 2), so 27 + the parity of r, as pybitcointools indicates.
So for even r, we get v = 27, odd r we get v = 28.
How is the value of v (27 or 28) important? Why is it necessary to have a v value? Also, why is it 27?