slightly disagree with people saying there is no benefit to parleys, the math works out the same if you don't account for odds movement, so if you anticipate odds getting worse then locking them up in a parley can be valuable, not to mention the time saved and the inability to roll over single winnings onto something happening simultaneously (like ignp mentioned) which happens a lot in soccer, basketball, gridiron, tennis etc. but this goes both ways, alternatively if you think the odds on something might get better than holding out and rolling winnings over onto the next selection can be a good idea too.
there's nothing about odds changing that's a phenomenon, it's 100% natural otherwise everyone is basically agreeing that the odds are right. there's many ways to predict movement such as public sentiment, comparing various sites to look for trends one way or the other, your own analysis of the event to name a few.
but yeah the emphasis on parleys with the general public is all psychological, casuals bet for fun and think they are being smart when they make some long-shot parleys which if you factored in the overround taken out of each selection means they are mainly getting terrible value. parleys only are worth doing if the odds aren't too juiced and you think they infer value. one way to get around the overround is to roll over winnings on exchange markets (if you have access) where the juice removed from the lines is minimal.