What is the Complex B for?
Pretty much everything.
B-complex vitamins are cofactors for enzymes. Enzymes do not work without them. Without enzymes, you do not convert chemicals from substrates to products.
Enzymes also use trace minerals, so not everything is dependent on B-complex vitamins. But pretty much every metabolic pathway (a series of related enzymatic transformations) depends upon one of more B-complex nutrients. [Source]