i think the reasons for it are historic.
when compulsory schooling became law in 1870, the vast majority of free schools were founded and funded by the church of england, in england.
so state provision extended and was grafted on to that.
so because they had been the main providers of any education to the masses before the 1870 act, the church of england got that clause inserted into the act.
and it's still there.
but obviously where the majority of the local populus is non-christian they do less specific assembly, or get an exemption from doing it at all, as in birmingham.
i've never heard of catholics walking out of traditional assembly, it was only jewish where i was, and some of them stayed in.