I know that a presoak for short development times is recommended often. But I don't understand why it makes a difference so I thought I'd ask. Here's my thinking on it:
Let's take tray development to make it easy. Let's suppose for some reason you measure your temperatures and do the math and come up with a development time of 3 1/2 minutes -- well in the range for which most people recommend a pre-soak. I know from watching the process in the light that if I put a sheet of 4X5 or 5X7 film in a tray that the entire surface area of the sheet of film will be under the solution in one second or less. Often, a lot less. Since the emulsion is essentially uniform in composition, it seems to me that it should all be moving toward the 3 1/2 minute "done" mark at the same pace, plus or minus one second or less. As a percentage of difference the non-uniformity of development time should be insignificant. (Less than the time difference between sheets that results from the few seconds delay in putting the second sheet in the solution, for example, if using a slosher, or much less than the delay into the stop and the fixer is shuffling multiple sheets.)
So why would soaking the film in water first make development any more uniform? It isn't like the emulsion surface is water resistant. Emulsions soak up water in a flash. I can see that if you are naturally inclined to not get the film under the developer promptly (for example, dropping it in the tray and letting it float around for 30 seconds before really pushing it under) that could lead to nonuniform development, but in normal processing, why does a presoak do anything more than add a handling step and a chance for scratching film when it goes in the developer?