Must? Nope
But you should,
Can just pour 250, 300ml bleach (for around 8kg size) and run that cycle monthly.. or buy those purposely made WM detergent. Or h2o2 stuff also can.
Else mold will grow at the back of the tube whr it's not directly visible to you and once a while, you will get those black flaky stuff on your newly washed clothes after a few years owning the machine and your clothes also smells bad