Your teeth don't a good job of grinding oats. They do a terrible job at in in fact. Most of the time, when you eat raw oats, they just get a little soggy and folded before your swallow them. Your body then has to jump through hoops as it were to process them. Not only does it have to expend extra energy and take extra time breaking them down, but more of it winds up wasted. This actually helps with colon health.
Additionally, when you eat raw oats that have not been soaking in anything, your body has to use it's own water stores to process the oats. When you boil the oats, you do that work for your body. If you eat them raw, your body draws water from typically under the skin in order to process them.
Thus eating them raw like that can help reduce subcutaneous water retention. But this is true as long as you don't boil them. Just dropping them in a shake for a minute or two won't process them, so you'll still likely get this benefit.