In the distant past, like 4 years ago, your only options for wide-range gearing from Shimano were an 11-27 or 12-27 cassette combined with either a 53/39 or 50/34 crank. People who wanted to climb often chose the knee-friendly 50/34 and 11-27 giving a gear range from 34.0 to 122.7 gear-inches. The 53/39 + 11-27 gives you a range of 39.0 to 130.1 gear inches, BTW.
Then somebody at Shimano ran the numbers and figured out that if you combined a 52/36 crank with an 11-28 cassette you'd end up with almost the same gearing as a compact: 34.7 to 127.6 gear-inches. As you can see, the 52/36 is a nice compromise between the 50/34 and 53/39: you still have the same low gearing for climbing, but a bit more top-end for hammering along the flats in a pace-line. Plus, your buddies who haven't run a gear calculator will still think you're a stud since you've got a 52-tooth chainring