Describe a real use case for VLANs. Please be as specific as you can. I$\text{'}$m struggling to understand and hoping that excessive explaining may help.

My prof. gave this example: An office supply company that has a waiting area for customers where it provides coffee and free WIFI to its guests. In the back office, it has a warehouse with supplies, a Sales dept, HR dept, and Accounting Dept. With this company, I could show how I would use VLANs to separate out the guest WIFI from the rest of the network and protect company data. Perhaps I could separate the HR dept to its own VLAN to protect employee personal information.