Importance of Body Transconductance MOSFET
For integrated circuit design the backgate-transconductance can be found using design documents or simulation. For discrete circuit design the source and bulk are commonly connected so that it can be ignored.

Backgate effect in particular occurs because the backgate is connected to fixed potential while the source potential can change. If source and backgate are at the same potential (e.g. both at ground) then it can be disregarded.

NMOS Source-followers with the substrate connected to ground show a non-linear behavior because of the backgate effect. When the source and the backgate are connected and the current-source is a high output impedance, source followers work much better.