A fork is most effective when it is forcing, such as when the king is put in check.
However, a queen fork can often lead to material or positional gain when the forked pieces are undefended, poorly coordinated, or when one piece is the king.
This example is from the first round of the FIDE World Chess Championship 2004 between Mohamed Tissir and Alexey Dreev.
This example is from the ninth round of the Clarin GP Final between Guillermo Soppe and Fernando Braga.
The only move is 41.Ke2 which enables a royal fork with 41...Nc3+, winning the queen.