There are a lot of bikes with cheaper rotors than 105 so I wouldn't worry about it especially if they are your first set of discs.
Above that level a lot of it's about weight saving, so more expensive manufacturing, and a bit of heat dissipation. Unless you're braking hard for a long time on a steep descent on a hot day you'll probably be fine - certainly better than rim brakes.
That said, rotors aren't very expensive so there's no harm in upgrading.