There are reasons to go with mismatched tires, but the previous owner may or may not have known these reasons. It could just be what was available as someone said.
That said, having the bigger front tire has some benefits and drawbacks:
Pro: A wider tire will have more grip, and this could make a difference in the wet under hard braking. Rubber pneumatic tires work more on adhesion than friction, and the rubber squeezes into the small voids in the pavement. If the surface were perfectly flat, it wouldn't matter what size your tire was, but a tire at lower PSI will engage more of these peaks and valleys and you can run a larger tire at lower PSI giving better grip.
Pro/con: A larger front tire will make your trail larger, and steering will feel more stable. "stable" could be replaced with "slow" or "sluggish" here, depending on how you like your bike to feel.
Con: A wider front tire has more of an aerodynamic impact than a wider rear tire. If your rims aren't pretty new, there's also little chance that their aerodynamics are optimized for a 28c tire so the impact would be significant, as far as wheels do impact your overall aerodynamics.
Pros/cons of a wider rear tire:
Pro: More comfort. Even at the same pressures a wider tire will usually absorb bumps better due to the increased air volume, but you can (and should) lower the pressure relative to what you'd run in the smaller tire. The rear tire, in my experience, has more impact on comfort than the front.
Pro: A rear tire is shielded somewhat aerodynamically, so its impact isn't as significant as the front, and it won't slow you down as much.
Pro: A rear tire will usually wear out faster than a front tire because of the force from the pedals going through it and the extra weight on the tire. A wider tire will have a longer wearing lifespan.
Con: Technically, your rear tire will have more grip possible than your front in cornering. Most crashes without a motorist involved have more to do with braking, however. Under braking, your front tire will always have more grip than your front, unless your tire is very big and your weight distribution is unrealistically low. This isn't likely to happen.