I've been to a lot of christenings lately. None of the families are in any way religious. The other children in the family were also christened, and outside of their christening, do not practice religion, attend church, etc. I have lost track on how much we must have spent on christening gifts which I can't really afford right now, but also don't like turning up without anything.
One family didn't want a christening but instead had a naming ceremony (for a 9 month old...who had been named 9 months ago). They received piles of gifts and in truth I did wonder if the whole thing was just for that.
The only reason I can think of is tradition within the family, but none of the family members are religious either and parents aren't christened