But some bacterium always lives in surroundings with the same concentration of dissolved substances as in their own cytoplasm. what structure do these bacteria not need? (fourth paragraph of subsection implies the answer)

Cell wall

The rigid bacterial cell wall protects them from osmotic lysis or rupture when they are placed in hypotonic solutions (solutes concentration outside the cell is less than inside the cells). The cell wall is a rigid limit to the swelling cytoplasm. Therefore, if the concentration of the solutes in the surrounding of the bacteria the same as its cytoplasm, in this regard the cell wall wouldn’t be necessary.