Based on your response, I'm guessing you're a little unsure of what is meant by offset and, to be honest, I'm not exactly sure what you're describing.

The offset of a vehicle's wheel is the distance between the vertical centerline (between the rims) of the wheel and the vertical plane of the hub-mounting surface of the wheel ...

If your new wheels have a 30mm (assuming negative here) offset it means the hub-mating surface is 10mm closer to the outboard rim of the wheel or 10mm further out than a stock 240 wheel. This means the wheel will sit 10mm further inboard and hence the recommendation of some 10mm spacers to keep the wheel/tire off the strut tower. You also want to keep the wheel/tire combination on the front as close to the stock position as possible as changing where the wheel sits in relation to the hub will alter the steering geometry. This isn't a huge deal on a track-only car but on a daily driver you don't want the negatives that come with messing with the offset. Been there, done that...

Many people add spacers to the rear of the 240 as the stock position places the rear wheels some 2" inside the fronts, meaning the rear tires do not follow directly behind the front tires and give the perception that the car is "dog-tracking" when viewed from behind.

Hope that helps...

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Dale