make better preflop decisions as it's one of the most important decision you'll make when you play. it's also what, for the most part, will earn/cost you a lot of chips in the long term. even the simple act of having a 3bet preflop range over a wide/loose preflop limp/call range with earn you more in the long term. just tightening up you preflop range in all positions will earn you more in the long term.
it doesn't have to be super complex either. just small changes will earn you more in the long term and provide you better long term results. it can be as easy as folding marginal suited hands in early position, like j9s or q10s. these hands are pretty and can potentially hit flops hard, but that's true for every decent hand. you also don't hit flops that often, you flop nuts even less often, and if you do hit a draw or top pair, there is still risk of you drawing to worse or still being behind. and when you do get hit, are you getting paid off enough to make up for the times you missed your draws or was drawing dead/got outdrawn? it's very unlikely.
now, what if you start folding these hands when out of position? well, the big positive is you save yourself a big blind or more if you like to limp/call raises out of position with mediocre hands. so, what good is 1 big blind? on it's own, not much. but over the course of an mtt, lets say you fold 20 mediocre suited hands in ep that you really wanted to play. well, that's 20 big blinds you saved. in an mtt, that's huge. that's basically a stack you just saved that you can put to better use in better spots.