Basically, PAL games on most old consoles tend to run at around 50 FPS, whilst NTSC games run at around 60 FPS. So the reasoning behind all these conversions people have been doing recently is that they get to play the PAL version of a game (which might have more content, bugfixes etc., or even just not have a NTSC version, like Terranigma), in a smoother speed than before.
I hope this helps.
(Also, first post, finally. No more lurking for me.