Well, I'm not a teacher, but this is how I see it.

In infant schools it seems to be used as a method of streaming children so that although one topic/method can be taught to the whole class, different work sheets can be handed to children of differing ability to help/stretch them whatever their needs.  It also makes it easier for teaching assistants to be deployed where they are most needed. 

In Junior school, it's used in much the same way for maths and english, but pupils return to their regular places for all other subjects so that all children benefit from one anothers differing opinions, strengths, idea's, etc and can mix.

Donna X