Reloading is the only way to find the 'sweet spot' of that particular firearm. You select a bullet and then compare various powder loads to see which one gives the best results. You may start off with a 115gr bullet and 3.5grs of W231 and shoot a 3" group. Then as you increase the powder by 1/10th gr it may drop to 2 7/8"...Then another tenth and 2 1/2" 5-shot groups...Then suddenly, at maybe 4.7 grs, you shoot a 1 1/8" group...Then if you increase any more, the groups start increasing again...So that particular powder load, for that bullet and that firearm, is the ideal load.
Commercial ammunition is a compromise to allow the best results in ALL firearms, not just the one you have. Only by reloading can you achieve all that that firearm can deliver...