Maybe if I want to get a more stable / consolidated price (closer bid / ask, esp for options) vs. placing limit order before market open. I have been looking for something similar.
A lot of reasons. For example, if your backtest showed that 10am always beat the open (even after accounting for crossing the spread). In this case you'd be better off using an aggressive algo set to start at 10am, or a more passive one of there is nothing special about the exact time of 10:00:00. IB offers plenty of those.