In a low-volume market, an investor could end up buying units at a price higher than what he/she would have wanted. On the other hand, you may be forced to sell at a price less than what you had hoped for. This is because there are no buyers (or sellers) willing to pay (or accept) the price at which you want to trade.
A limit order protects an investor from both the above scenarios.
This is extremely important in the case of an ETF because of inherent liquidity issues and a difference in the indicative NAV and the Market NAV.