Why Topo? because you want to create an edge/poly layout that mimic the way the muscles stretch.
Face - Reference Images
200px From http://forums.cgsociety.org/showthread.php?f=25&t=38469 - "I've marked in red some important areas - the wrinkles at the corner of the eyes are very hard to get right if the topology is wrong. The big nasolabial fold is tricky because on some people it connects a little higher on the chin in the smile, but I think I got a good average placement here. The little wrinkle at the corner of the mouth is sometimes overlooked. In many old people it does seem to cross the nasolabial fold like that. The bridge of the nose actually displays this surface shape in some people, and I believe we all have the same structure there, just many of us (like most asians, and me) don't show it. (The top of the nostril is one of those areas I might decide to change later, most people have a line there, and modeling it diagonally across a quad is a big pain.)"
- Japanese - Great Video Example: https://www.youtube.com/watch?v=LFqopkUTO0Q
- Detailed explanation esp about eyes: http://site.rukout.com/resources/3d-facial-modeling
What not to do
- Image from: https://www.youtube.com/watch?v=k_S1INdEmdI