What are the benefits of mixing foundation and BB cream?

These days, I've noticed many BB creams coming out that aren't grayish and have a base-like color. So, I usually alternate between them. Is there any benefit to mixing a foundation with a BB cream that doesn't have a grayish tint? I do occasionally see people who mix foundation and BB cream well; why is that?

Do you know if mixing the two products improves longevity or what other advantages there are?