If your computer is not connected to any other computers and doesn't have a modem, the only way anyone can access your computer's information is by physically coming to the computer and sitting at it. So securing the room it's in will secure the computer. As soon as your computer is connected to another computer you add the possibility that someone using the other computer can access your computer's information.
If your network (your connected computers) consists only of other computers in the same building you can still secure the network by securing the rooms the computers are in. An example of this would be two computers sharing the same files and printer, but not having a modem and not being connected to any other computers.
However, it's wise to learn about other ways to secure a network of connected computers, in case you add something later. Networks have a tendency to grow. If you have a network, an intruder who gains access to one computer has at least some access to all of them.
Note: Note that once someone has physical access to your computer, there are a number of ways that they can access your information. Most systems have some sort of emergency feature that allows someone with physical access to get in and change the superuser password, or access the data. Even if your system doesn't have that, or it's disabled, they can always just pick up the computer or remove the hard drive and carry it out. More on this in the physical security article.