Why does map take lambda as a template parameter

[xtreak via Digitalmars-d-learn]

map takes lambda as a template parameter and so does filter and 
many other functions. Sometimes they take something other than 
lambda as a template parameter. Eg. In case of to!int("5") int is 
a type and hence might need it as a template parameter but why 
does map and others take it as template parameter.

Adam D Ruppe pointed out in IRC it helps in inlining and 
optimization. Is there a thumb rule to decide this so that my 
functions too can benefit the performance and hence I could 
structure and understand my code better.