Hatcher writes in his book "Homology calculations are often simplified by taking coefficients in a field, usually $\mathbb{Q}$ or $\mathbb{Z}_p$ for $p$ prime".
I wonder why one only consideres $\mathbb{Q}$ as field of char. $0$. I have seen proofs where one takes coefficients in $\mathbb{Q}$, but I have seen none where one takes coefficients in $\mathbb{R}$ or $\mathbb{C}$. Is there a reason for this? Does coefficients in $\mathbb{Q}$ give more information than coefficients in other fields of char. $0$? Or is it simply a convention to take $\mathbb{Q}$-coefficients?