Basically any office with PC's will have them all connected to the network via cat5 (or better ie cat6 etc) so the PC's can talk to each other, servers, printers etc etc etc. Just about all the equipment in the office can use this cabling, even modern IP Telehpones via bunch of network equipemnt such as routers, switches, patch panels. This will also be the way any office PC connects to the internet and e-mail.
As already explained cat 5 cable is very cheap to install. So in a HC you can use it to pipe AV around the home without spending a fortune on ever changing cabling requirements.
A better idea of what you can do with cat5 can be seen by looking at the baluns you can buy to convert your AV signals. Have a look
Here for some examples. They are not cheap, but probably cheaper than wiring up loads of different cables throughout a house.
Mark.