It's about trade-offs. You can roll your stats to try to get a 16-18, but you take the chance of getting a 3-7 in another stat. You also take the chance they you might end up with a net +3 or +4 modifier bonus rather than the +6 from the array.
Also combined with the 20 cap to natural stat improvement levels the playing field at mid and high level play. If you have a natural 18 through rolling and another character has a natural 15, the first character (with a +2 racial modifier) is as good as he's ever going to get without magic at that thing. The character with the 15, however, can catch up in just a few levels (+2 racial bonus, +2 stat bonus at 4th and +1 at 8th (or 6th for Fighter)).