For example, an Elf who grows up in an Elf culture will enter level 1 with Gish experience. The Fighter class needs away to express gishiness, even at level 1, for narrative reasons. Fight Style works well to do this. For example, the Fighting style itself can swap the Heavy Armor for always-on Mage Armor, plus add a cantrip.
Similarly for the Paladin. The Paladin is a gish. Certain character concepts need to start off gishy at level 1, and the Blessed Warrior Fighting Style did this well enough but needs to happen at level 1.