Sourcing animals from rescue centres has several advantages. At least in Asia, where I have visited several, rescue centres are typically, but not exclusively, places doing their best under difficult circumstances. The demands on them may be higher than they can support, and they usually don't have the funds to make much needed improvements or expansions.
If a zoo buys animals from a centre (or establishes a financially beneficial relationship with one in exchange for occasional imports) at least four good things happen:
The zoo gets a new species, which may be exciting enough to boost visitor numbers.
The centre receives a nice cash injection, and frees up space to rescue more animals.
The animals in question will quite likely be going to much better enclosures, although climate and diet may be issues.
Depending on why the animals were in the centre in the first place, there may be a very interesting, and educational, story to be told, to the benefit of both the zoo and the visitors.
As an additional maybe, if an initial pair do breed freely, other zoos may be inspired to follow suit, thus establishing the species outside its home range anyway.
That last comment not withstanding, all I'm trying to show is that just because you don't expect animals to be founders, doesn't mean there is no value in importing them.
(I should also point out that this is based on logic, I know nothing really about the interactions between institutions)
