The description does not mention an on/off button, but something else "With reset fuse button".
This is a fuse built into the power strip to prevent it from being overloaded. That is why the black ring around the red button should also say "Press to Reset", so that the fuse can be reset if it has tripped.
Is either a fuse or an on/off button. However, the picture is probably wrong because the Brennenstuhl website shows the product (same item number) without a button.