Olive trees provide opportunities nothing else does right now. First, they can be harvested as wood if necessary, but they can also grow olives. Olives are useful as food in and of themselves, but it should be possible to build a press (hand-cranked and electric-power hydraulic version) to make olive oil. The reason olive oil is important is it should be possible to process it into plastic, and the plastic combined with steel into plasteel. This allows you to move from neolithic to space-faring technology without the need for trading. The press would be useful for making other things too, like cheese from milk (properly pressed cheese would remain edible a long time, like pemmican), and whiskey from corn.
Olives would only grow in, like, 2 of the possible biomes of Rimworld.