It is common practice to use non$-$routable IP addresses for internal networks. What would be a MAIN reason for you as an IT administrator to use internal non$-$routable IP addresses?

To solve the problem of $4\mathrm{.}3$ billion IPv$4$ addresses shared among $10$ billion devices

So VLAN can isolate networks

Non$-$routable networks is the only way to connect Windows PC clients

To solve the problem of hackers getting onto your network