I have always thought you subtracted the AC from the THAC0 thus getting the goal number for the roll. The bonuses (usually not being that impressive) only need to be considered if you failed but got really close.
The problem was, the modifiers can get pretty impressive.
Be Elf. +1 to hit with sword. +1 to hit with 17 Strength. Possible +2 to hit from Bladesong Fighting Style. +1 to hit from weapon specialization. Maybe +1 to hit from exceptional quality sword.
Now add some magic.
+1 sword, +3 vs. regenerating creatures. +3 (actually +2) to hit from Gauntlets of Ogre power. +1 to hit from Bless.
Or hey, instead of Bladesong, how about two-weapon fighting? +1/+2/+3/+4 flametongue in one hand, +2 short sword of quickness in the other.
I have a Samurai with two different Strength scores, normal and 18/00 when he uses his kiai. Rangers occasionally get a +4 against certain foes, and Dwarves and Gnomes have +1 against certain foes.
An evil character fighting a Paladin suddenly has a -1.
What if your DM uses weapon vs. armor type optional rules? Then each weapon potentially has like 9 different bonuses based on the armor in question!
Oh, you want to make a called shot? That's a -4 to hit. Situational modifiers- a rear attack ignores Shield AC and gives you +2 to hit- but it's +4 to hit if you're a Thief. Attacking with two weapons can impose -2 to hit with one weapon and -4 to hit with the other...or less, depending on Dexterity. Defender invisible? -4. Defender off-balance? +2.
(Hey Anakin, do you remember what bonus Obi-Wan has for the high ground? It's +1!)
Speaking of Thieves, for some reason, if you're a multiclass Thief, you might have to keep track of two different Thac0's- say, your Fighter one, and your Thief one with +4 to hit when using Backstab.*
*I don't believe this was stated in the PHB, though I saw it as a house rule at some tables, and it was eventually confirmed in Dragon #243, which clarified how multiclassing was supposed to work, which most of my DM's ignored, as they'd come to their own conclusions about multiclassing by that point, lol.
Some monsters have multiple AC's, like the Bullette, depending on where you attack it. Some enemies have -4 to hit Dwarves.
And this is far, far from exhaustive.