My parents bought plots for them and my brother and me a while back. Granted, it's weird to go out and actually stand on your own gravesite, but it really is a responsible thing to have (just like everyone should have a will - even when they're in their 20s; it's the responsible thing to do regardless of how morbid it may seem at the time). Things like that need to be bought in advance (for obvious reasons). And, if a family wants to be buried together, you really do need to have the plots bought in enough time to get them all in a row or section together.
While kind of an odd gift right now, I can see how thoughtful it was for them to give it to you. This really is a nice thing to do for someone, believe it or not.
(And, if in the future you decide you want to be buried elsewhere - like if you move and decide you want to be buried in the new city or state - you can sell the plot you have now. You're not completely committed to it where it is now, but it is a good investment to have.)