Single vs Multiple Speed Gearbox

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In the past, my team has just focused on trying to do alright at events. Recently, our team has kind of moved up and have become better, so we have been looking at more types of drivetrains. Ive looked online and here comparing different gearboxes but I haven't seen much about why a two speed gearbox or shifting gearbox is better than a single speed gearbox. Could anybody tell me what the advantages are and if it's worth the complexity and space it takes up? We have been looking at the WCP DS just because we've heard that it's probably one of the best gearboxes for FRC.