its not just more lift..thats simple..its more lift earlier (intake lobe/tapit) or later (exhaust lobe/tapit) at points you cant achieve with a flat tappit...
you can get aggressive loge ramps with a flat tappit but at some point you put too much side load on the lifter..
and the lobe sweep across the tappit face you will find a numb spot...where the lobe moves/crankshaft degree of rotation moves..ie piston moves..
but the valve no longer does...
this is why the roller is superior.