In ARM VPF, why does the exponent part should add 127(in 32-bit) to represent the exponent?

In ARM VPF, why does the exponent part should add 127(in 32-bit) to represent the exponent?

In VPF 32-bit mode, a float number is store as follow:

31    30... ...23    22... ...0

S    Exponent    Mantissa

As for +0.5, the exponent is -1 ( +0.5 = 1*1*2^(-1) ). But in fact, the Exponent is stored as 126(-1+127), so I want ask why we should add a 127 then store the number in Exponent?