You don’t get any benefit from having your resistances over 75%, unless you also increase your max resistance cap.
Increasing your max resists can be very beneficial and reduce the damage you take significantly. You could theoretically reach over 100% in some max elemental resist and become completely immune to that damage type, but in practice it’s pretty hard to achieve.
The simplest way to get 5% to max Lightning, Fire or Cold resist is to use Purity of Fire, Lightning or Ice auras. Ruby, Topaz and Sapphire flasks also increase your max resists by 10% while their effect lasts. For bosses like Piety with a strong lightning attack having 90% resistance would substantially reduce incoming damage: going from 75% to 90% lightning resist would not reduce lightning damage you take by 15%, but rather by over 60%! So being overcapped while increasing your max resists can be incredibly strong.
A unique flask, Divination Distillate, also gives you 6% to all maximum elemental resistances during its effect, and Elemental Adaptation node in the skill tree (near Marauder starting) gives you 2% to all max elemental resists all the time — well worth taking if you’re near it.
Lastly items that can increase your max resists are Rise of the Phoenix (8% max fire), Aegis Aurora (5% max cold), Kaltenhalt (5% cold), Safell’s Frame (5% all max resists). Most of these have downsides so they aren’t practical to use all the time but if you need specific elemental resist they can help a lot.