I've actually seen it argued stirrups are on the whole actually more useful for mounted sword-swinging than lance-charging. Not entirely convinced that checks out, though, at least unconditionally - Medieval Middle Easterners and the Moors were anything but shy of ferocious hand-to-hand combat despite riding with short stirrups, while the distinctive equestrian equipement of period European heavy cavalry (the light horse didn't normally use it) seems to have been very intimately connected with their focusing on the couched-lance charge as the end-all be-all of mounted spear use. (The technique was known and used in the Middle East too - around the Crusades the Arabs apparently called it "Syrian attack" - but wasn't similarly overdominant, doubtless due to the less "linear" character of field combat in the region.)
On a more general benefit, stirrups apparently rather reduce rider fatigue as he doesn't need to grip the horse with his legs all the time.
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