It only applies to warriors. At 18 strength a further roll is made on 2D10 (one for the first digit one for the second) giving a value from 1-100. (18/00 being 100). You got additional bonuses based on this. I find weight to be more useful that the damage and THAC0 bonuses to be honest though.
https://baldursgate.fandom.com/wiki/StrengthThere is a tome in chapter 6 that will take you to 19 STR, so if you are going for the whole series then it doesn't really matter.
As an aside, a pure fighter is pretty poor late game, though great in BG1 one. If you enjoy a mage and you're Human you may want to consider Dual classing to a Fighter>Mage at level 9 (13 likely too late for a first timer). This is assuming you have enough INT.
Mages start very poor, but they are godly by the end game and will rarely take any damage with all their protection spells. Having a few levels of fighter before dual classing mitigates their worst downsides with little loss.