To be more concrete, imagine another user places a malicious script into the file "/tmp/ls". If later someone else wants to look what temporary files are there,

To be more concrete, imagine another user places a malicious script into the file "/tmp/ls". If later someone else wants to look what temporary files are there, typing "cd /tmp" and then "ls", that person would suddenly execute the malicious script without even noticing. This is even worse if that happens to root, which is why this advice is especially important for the root user. Also, note that for often-used self compiled binaries, $HOME/bin is a good place. On some systems, $HOME/bin is already added to the user's PATH by default, so you just have to create that directory and put your favorite binaries or shell scripts into that.