Chainstay length is BB to rear axle distance, direct. Rear centre is the horizontal distance between the BB and rear axle. The projection of BB on the ground to the tyre contact patch if you will (and same for front centre). This is affected by the dynamic sag both front and rear as it is affected by the inclination of the bike frame. Chainstay length (the changes of it) affects pedaling performance (antisquat and kickback), rear centre length affects front rear balance. Though technically for that the axle to centre of gravity distance would make more sense than the distance...