Congratulations, some previous owner did an upgrade and you're the beneficiary! The 15" rims give you a bit more ground clearance and a lot more variety in tire selection (the ability to select higher load tires being).
I borrowed a friend's little trailer once that had 12" tires on it. They were old and I blew two out on the interstate. Had to drive 75 miles to the nearest large town to find replacements, and then they only came with tires and rims. If the trailer had 15" tires, I could have bought replacements locally.
And, welcome to the forum!