It depends on the type of retaining wall. Generally, gravity walls (ones that are stable due to the weight of the soil or rock behind them) don't need drainage, whereas masonry and reinforced concrete walls do. This is because gravity walls rely on their weight to keep them in place, and if water accumulates behind them it can weaken the wall and cause it to fail. Masonry and reinforced concrete walls, on the other hand, are not as strong as gravity walls and are more likely to collapse if water accumulates behind them. So drainage is necessary in order to prevent water from building up behind these types of walls.

It depends on the retaining wall. A gravity-type retaining wall does not need drainage, but a reinforced concrete or masonry retaining wall usually does.