Main reasons is probably the upgrade of chip. With new card you should get better newer 'HW' security updates, etc.
The second one is also connected to security - bank has a chance to re-check your identity.
And also business opportunity, to offer new products, services. etc.
I guess historically it was because of security. When you were providing a copy of the card with every transaction you wanted to change it often.
Nowadays I believe it is because of the root certificates expiring. So you need a new chip with updated certs on it.
Yes.
One of original reasons might be also additional security info layer which you need to know (not only number, but expiry date).
