A water-saturated emulsion takes up the developer more slowly (and hence, more evenly if the film is immersed in the developer over a longer period of time, e.g., when it takes a long time to fill a tank). The increase in developing time needed to compensate for this is likely less than 30 seconds, though, so a pre-soak shouldn't make a huge difference in development time.
See above for one example of how a pre-soak can be beneficial.
My primary reason to pre-soak is so my sheet film does not stick together in the development tray and so that initial agitation is easier. It takes a few minutes for the emulsion to become saturated with water. After that, shuffling sheets is much easier. A dry sheet of film, immersed too soon after another, can stick to the one below it like they were glued, and soaking them apart takes some minutes. I don't need that to happen in the developer... I let each sheet of film sit for 15-20 seconds in the water bath before adding another; otherwise, they stick. After soaking for a couple minutes and then shuffling through the stack a time or two, they are ready to go to the developer tray.
Some use a pre-soak to get tanks, drums, reels, and film up to processing temperature before starting development. This can be useful for repeatability and process control with higher-temperature processing, e.g., color processing.
One caveat about pre-soaking some films. There may be some films (Ilford, I believe, and maybe others) that have a surfactant incorporated into the emulsion to help with developer absorption. Pre-soaking these films for too-short a time results in an uneven distribution of the surfactant and can result in streaking. The problem goes away if the soak is long enough; somewhere in the neighborhood of 4-5 minutes if I remember correctly.
Exactly! If you need to pre-soak your film for a good reason (like I do), then you can be assured it won't hurt a thing. If you don't, then immersing it into the developer quickly when dry works just fine.
Doremus