In short they 'added' it because hybrid jobs like BLU could not land Sudden Lunge's Stun or many other physical spell additional effects because swords don't normally have magic accuracy, and a level 99 player fighting a levl 110 mob with their lv110 magic evasion is very hard. Additional Effects from grips or relic weapons weren't procing because the proc rate is modified by magic accuracy skill.

Let's take Polearm for example.

Leg Sweet is a WS which adds Stun. The chances of this stun landing are based on "magic accuracy skill". This value is increased at the same time as weapon skill. That's why a SAM with 0 Polearm Skill will probably not land Stun on a EP/DC mob while a DRG with 400+ polearm skill will easily. Since we're capped at 99 and weapon skill level isn't increasing, they added m.acc skill+ to weapons. This will allow additional effects from weapons, grips, weaponskills and secondary effects from physical blu spells proc on mobs that are level 100 and above when being hit by players capped at 99.