what is the academic sense behind the oxford reading tree?

[DimpledThighs]

DD has just read one to me and they really seem to help her reading but I cannot see why.

Any clever teacher types able to fill me in?

Is it the phonetically spelt words, the repetetion or what?

They seem to through in a new word on the last page - is this intentional or coincidence?

I have wondered this for a while.

My daughters school also have a selection of reading schemes though we really don't rely on the books she brings home from school as we feel she is on a much lower level than she should be. At home she reads M&S bright sparks books, charlie and Lola and many others.