In a rough way, a category of motives over a field $k$ with coefficients in a field $K$ gives a universal cohomology theory with coefficients in $K$ for algebraic varieties defined over $k$. I had the impression that the choice of coefficients $K=\mathbb{Q}$ was the most standard one and at least the most natural one($\mathbb{Q}$ is the smallest field of characteristic zero).

But in this video conference:

https://www.youtube.com/watch?v=0M-jXPi_t1I

around the time 8.00, Kontsevich says that the "ultimate coefficient field, that you can use in any situation" is $\mathbb{Q}^{CM}$, the union of CM fields, i.e. of imaginary quadratic extensions of totally real number fields.

I don't understand why it is true so my question is:

Why is $\mathbb{Q}^{CM}$ the "ultimate" coefficients fields for motives?

More precisely:

1)I can understand that a bigger coefficient field than $\mathbb{Q}$ can be useful but I would like to know concrete examples of that.

2)I don't know why $\mathbb{Q}^{CM}$ should be enough to cover the eventual answers to 1). I know the usual appearance of CM fields in the theory of complex multiplication of abelian varieties but I don't see a direct relation with the question.

Remark: I am deliberately unprecise on what I consider as "motives". If the answer depends on "details" of the notion (pure/mixed, equivalence relation on cycles...), I would like to know it for the various versions.