I can't speak to the reason InfoBlox was put in this environment, nor can I speak to the management of DHCP and DNS on current versions of Windows as I stopped doing Windows admin years ago.
We use InfoBlox for Internet-facing DNS and internal DHCP. Having the single admin interface for these services is more convenient that having to manually modify a zone file correctly (i.e. w/o typos, remember to update serial num, ..), or to manually split DHCP scopes between servers or globally update scope options.
A dedicated appliance for these functions streamlines the support of the service. We contact Data Center Services to get the appliance physically installed, cabled, and powered up. We have responsibility for the box and everything it does from that point forward. Loading another application, such as BIND or ISC, on top of a general server would require adding the server admin team for the relevant OS, Storage team for storage, and more time for troubleshooting and fault isolation. Appliance solutions streamline support down to a single phone call if the application stops working as expected.