Question for Agnostics: If God doesn't exist, or is unknowable, then why is it important to so many people to participate in church/synagogue/religion ?

I’m not approaching this from Marx’s perspective that religion was to subjugate the masses—because most of us alive today have never been subjugated with religion by our leaders. Nor am I going with that its ignorance or superstition, because every doctor I know is quite strong in their Christian, Jewish, or Muslim, faith —as have been many scientists, astronauts, etc.

My question is, assuming that God doesn’t exist—why do so many altruistic endeavors take place through religion (such as church health centers and clinics, aid to the poor, missionary activity, etc etc.) —when such endeavors could easily take place WITHOUT belief in God (but seldom do) ?

And, why do so many adults who find God largely irrelevant (whether they believe in God or not)—still bring their children to church, even when they themselves do not participate much?
They may not find God irrelevant but do not really get deep into it—and seem to want to expose their children in the work and life of the church nonetheless.