Perhaps you've tried something like the following:
```bash
sudo echo "hi" > /etc/test
```
Only to receive back an error
```
bash: /etc/test: Permission denied
```
This is because the `sudo` part only applies to the echo command. Bash which is the process that takes the `"hi"` from the `echo` standard out and writes it to `/etc/test` is still under the unprivileged user.
The command `tee` takes whatever is in standard in, and writes it to the filename specified. Since we applied `sudo ` to the `tee` command, it now has the necessary permissions to write to a privileged location.