The usual reason for setting static IP addresses on the router instead of on each device is that by default every device is already set to DHCP, and all of the management can then be handled in one place.
The other is that assuming that at some point you change your router. The replacement hands out addresses of 192.168.1.x and the original handed out 192.168.0.x Any device that you had set on the device itself with a static IP would now be unable to communicate when connected to the replacement router. By doing the management on the router itself, all devices would still work (albeit not on the IP address you want), and you can simply adjust the settings there, rather than on the device which may be a pain to get at if you can't access it due to the wrong subnet.
Beyond all that, I'm a fan of always having static IPs for every device that's permanently connected (ie: ethernet) and only utilizing DHCP for mobile devices.
Changing the pool of available addresses for DHCP is so that any static addresses you may set do not possibly conflict with an address being handed out via DHCP. But, truth be told I've never seen a router that ever hands out a conflicting address, and if I did I'd probably immediately replace the router as it is too completely brain-dead for me to trust it to do anything right at all.