This is because Unix/Linux tries to represent everything as a file/directory in the local filesystem (even things like hardware and states of the system).
So this also counts for remote directories:
After mounting a remote dir on a local dir you will be able to read/write from/to the local dir but the effects will actually happen on the remote dir.
It also makes it way easier for software to handle files on remote servers because it can use them as if they exist locally.
Example: In my
/etc/fstab file (the file containing all information about how the system should mount stuff) on my desktop I have a line:
//secrethostname/vanalles /mnt/targasmb cifs user,noauto,guest,ro 0 0
secrethostname is actually another name, i just don't want to post it online...)
When I now run
mount /mnt/targasmb on my desktop I will be able to see all files in the
vanalles dir on
secrethostname in the directory
/mnt/targasmb on my local system. I will also be able to able to read them as if they exist locally.
cifs user,noauto,guest,ro 0 0 provides the system more info about what the mount is and how it should be mounted by default. The details are for another question.