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Yes the extra tanks where to make sure there was enough reserve air for multiple brake applications and in the case of vehicles which were used off road such as logging trucks they where common to have more air tanks dedicated to the suspension systems one per axle to help with the extra air usage by the ride control valves over uneven terrain the alternative to this was to use ride height valves with a damper to slow their reaction time to changing ride height. Remember that air suspension was considered an accessory and would only receive air after the brake air tanks where filled above 70 psi requiring separate tanks to maintain the separation . On trailers such as a low boy it was common to have dump valves to lower the deck for loading ,these systems often used larger or more air tanks with relay valves and larger air lines to the air bags to help speed the recovery of ride height when loading was complete as this operation would use more air volume than could be supplied from the tractors air compressor quickly .