VLANs are a way of segregating traffic while sharing the same physical wires. The alternative is separate physical wires for each LAN (guest vs. staff).

If you don't do one of the above, then it is really easy for a user on the "guest LAN" to just reconfigure their own IP parameters to put themselves onto the staff LAN, ignoring what the DHCP server tells them to do.

It's a security thing.