You are EXACTLY right! The propeller is only at its maximum efficiency at one one speed for a given pitch. There are three modes for which a change-speed gear-box is desired but the one case that generated the most intensive development activity concerned the USAAF's determination at the close of the 1930's that a bespoke, ultra-high output (5,000-8,000Hp) 'bomber engine' was required for super-heavy, long (very long!) range bombers and transports. These engines would typically have low specific outputs due to their huge displacements so they required maximum thrust for take-off and climb-out but once settled in at cruise altitude the power gear-ratio was simply too high to maintain propeller rpm's once the engine was throttled back to lowest sfc point on the rpm range so a second optimal gear ratio was necessary (torque to spare!). Blade pitch change was still available on both ratios. gear-box fabrication was undertaken by Merz Engineering, Ohio. For further details, consult GOOGLE patents, Wright's designs at least, are there.