What are the arguments for and against the existance if zoos?

Please note, I am not looking for a great deal on the ethics side of
things (animal rights etc) but more for the scientific arguments. i.e.
function provided in education, research etc.

Zoo's do help us understand the complexity of animal life, but the
reduced living space does alter there behaviour as well, e.g. lions in
the wild will move around, and will chase there prey, but in a zoo
they must remain in a set place and climbibg a tree is the farthest
they go for a meal, and that's if the zookeepers bother to do that.
What I think I'm trying to say is zoo's are benefical, but to truely
understand an animal it needs to be obsirved in it's natural habitat.

Zoos are saving a few species from extinction, working together to
avoid genetic decline due to incest.
That sounds like a nice thing to do, but can't make much difference to
the individual animals.  Humans do it 'cause we have a collective
guilty conscience about killing off the passenger pigeons, and most of
the buffalo, etc.
And it's good to have a place to show the kids that milk doesn't come
in cartons; good place for dad to go with them Saturday morning, maybe
forcing him to tell them about the birds and ... - probably the
monkeys.
Oh, and in some parts of the world, zoos can be a source of
religious/medicinal (?) material.  I have a photo from the Delhi or
Calcutta zoo of a sign that said that elephant or rhino droppings
could be collected/bought at a certain time each day.  (No laughs or
offense intended.)

I worked for a short time at the Bronx Zoo. One of their primary goals
is to serve as a breeding facility for endangered species. In addition
to the animals that you see on display there hundreds of other animals
in facilities throughout the US that house breeding populations. These
animals are eventually reintriduced into wild populations.