Plenty
link of watches get by with just one mainspring barrel, but it's hard to argue with the principle that two are better than one. Mostly
link it's about extending the power reserve, especially if the watch is running a complication like an energy-devouring chronograph or, say,
link a triple-axis tourbillon. Inserting two mainsprings is like creating a larger gas tank that can keep you on the road longer at potentially higher speeds with more stops and starts.