When one levels up and gets an increase in maximum hit points, one can either roll or take the average roll rounded up. Statistically, you're better off with the non-random method.
For ability scores, the standard array is 15, 14, 13, 12, 10, 8. Using the point-buy system yields scores in this same basic range. But, according to AnyDice, the average rolls for 4d6-drop-one are 16, 14, 13, 12, 10, 9 — slightly better and, crucially, providing the likely possibility of an 18 to start (with a racial bonus of +2) and a 40% chance of starting with at least one ability score at 19 or 20.
It seems odd to me that the designers didn't go the same route as with hit points: make the predictable choice be rounded up from the average. This answer delves into the math and makes the same basic point — and ends up wishing that the designers had picked a slightly higher standard array.
We can all speculate on the pros and cons of this; I'm curious instead about designer intent and official references and commentary.