Oats are very high in phytic acid, so that is the reason you would want to soak them before making oatmeal. I have a post on my blog talking about anti-nutrients in nuts and seeds, so check that out for more information. Anti-nutrients, including phytic acid, in grains, legumes, nuts and seeds steal nutrients from your body when you consume them. This could be one reason why so many Americans are nutrient deficient. Most Americans are not taking the time to soak, sour or sprout their grains, legumes, nuts and seeds.
The nice thing about soaking oats is that is takes very little effort to do. It could even be one small change a person could make to their diet to improve it. When oats are soaked, the nutrients in them are much more available because at least some of the anti-nutrients have been neutralized. Another benefit is that you are satiated longer. Many people report that they are hungry just a couple hours later after eating unsoaked oatmeal. When changing to soaked oatmeal, they notice that they stay full longer.
Most people who soak oats use acidic mediums like apple cider vinegar, liquid whey, lemon juice or yogurt as the soaking medium. I have heard that a small percentage of people who soak their oats use rye or buckwheat (if gluten free) as the way of neutralizing the anti-nutrients. The reason for this is that rye and buckwheat have a high phytase content, which is even more effective at neutralizing phytic acid than a typical acidic medium. For this reason, I use both methods in order to neutralize as much phytic acid as possible. It is up to you if you use both or just one method.