What is the function of oil in baking/roasting? Oil is almost always used when cooking vegetables or meat at high temperatures in the oven, but I'm not sure what it actually does - it obviously provides flavour, but there seem to be other purposes.

Surface water prevents the maillard reaction from occurring because the temperature cannot exceed 100C. I presume that oil does not have this same property? How does oil aid in crisping the surface? Does it do anything else? Does it affect the form or speed of heat transfer?
(Liquid) water cannot exceed 100°C because that it's boiling point, and any additional heat applied goes toward the latent heat of evaporation needed to turn it into a gas (steam, which can then be higher than 100°C).

Oils have a boiling point much higher than water, and a point lower than that (but still much higher than water's boiling point) called the smoke point which you do not want to reach (as it causes dangerous break downs of the oil).

Oils aid in crisping by facilitating heat transfer. They are liquid so they have good coverage, and they sort of "stick" to the surface of the pan and the food. Excess drips off, but there's still a layer.

This layer has good coverage so it provides more contact between the food and pan, facilitating heat transfer.