It's just what you're used to. Offset is nice because it doesn't change with respect to the width and the centerline of the rim. Ie, if you have a 17x8 and a 17x9 that both have a 0mm offset, the centerline of the rim is in the same place when it's mounted on the car, the extra width is split equally between the inside and outside of the wheel (but the backspace has changed from 4.5" to 5"). If you have a 17x8 with a 4.5" backspace and a 17x9" rim with a 4.5" backspace, the location of the centerline of the rim has changed and the wheel will not be centered in the wheel well anymore. All the extra rim and tire went to the outside, so the clearance to the fender changed by an inch while the clearance to the suspension remained the same.
Both backspace and offset are important to know. If you change the width of the rim and keep the same offset the wheel will still be centered, but you need to make sure you still have enough clearance on both sides because the extra width is added to both sides. If you change the width of the rim and keep the backspace the same you know it will clear the suspension, but all of the extra width goes to the outside so the fender clearance has changed dramatically.