lets strap a beautifly on the experimentation table and start dissecting it.
5 sp.atk IVs, adamant nature, 80 sp.atk EVs --> 207 spA
15 sp.atk IVs, bashful nature, 120 sp.atk EVs --> 250 spA (if you do not EV train, you will not go above 120 EVs easily. spA is scarce. almost always, your special attacker will be stuck having like 150 EVs on attack, and less than 100 on spA)
31 sp.atk IVs , modest nature, 252 sp.atk EVs --> 328 spA
experiment 1 happens much more often than you think, if you just catch the first pokemon you find and stick with it.
experiment 2 happens often enough, but from my personal experience, it is on the rarer side, despite its absolute mediocrity
expeeriment 3 ... well ...
an optimal or close to optimal beautifly will deal 30%-60% more damage than the beautifly that 80% of casuals would normally get.
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now if you take experiment 1, which you can very likely get, (see? im not even promting you to go IV hunting) which is arguably awful, and do simple stuff like changing its nature to modest when you get access to psychologist, and take 2-3 pokecenter trips of EV training at woobats when you get access to power items (how much time can that take), your 207 spA will become 300.
thats an almost 50% increase in your damage. that means, that instead of soft-resetting 20 times on charlotte, because your attack leaves her pokemon with 30% of hp remaining, you might only need to soft-reset 5 or less times. then think about the total number of situations where a 50% damage increase will save you precious time, from seconds, to minutes (because things went south and you needed to soft-reset or if you are a masochist, white out and go back).
so yeah, you invest 2-3 pokecenter runs for each of your team members and maybe some psychologist visits for the nature *once*, and you suddenly have a much more functional team, which might save you more time than you invested, plus the added benefit of actually feeling badass.