Sometimes, people may put in a lightened hammer spring for easier cocking, but the original handspring might be a tad stiff, so as the hammer falls, the hand spring, being stiff, slows everything down, by requiring more downward force to compress itself, than what could be delivered with the lightened hammer spring, which can result in lite primer strikes and/or failure to fire.
So that's another reason people go for the coil spring conversion.
I think @Coffinmaker explained it to me like that one time.
I noticed the difference when I DIY'd the coil spring conversion on my Uberti 1860s a number of years back.