It's one extra hit AND +11 points of damage(+1 damage for 11 hits) over 5 fights. The key is that damage is so low that it doesn't take much to = 20% more, so it sounds impressive, but is pretty irrelevant.
irrelevant to you, but obviously not irrelevant to many players.
and yes, 20 damage over 20 rounds a day spend in combat at low levels might not seem much, but it is difference between 90 and 110 damage for a greatsword user. Or in relative terms a 22,2% increase in damage.
But problem is, it is you main job in your party and having your int or cha going from 8 to 10 will NOT compensate for loss of 20 damage per day, if you care about combat utility of your character.
lets, raise levels a little and change class, but keep it still simple for calculations.
So, you are 8th level warlock(just got your 2nd ASI), and you get your nice "20" in cha.
or you have only 18 because of
reasons...
And you do not like flashy spells or control spells. You just like to take bunch of d20s every turn and make attack rolls.
So, you invest in you eldritch blast invocations and run around with you beholdering from 4th level Summon aberration.
and let's again use 20 round of combat per day:
with 18 cha;
EB:
+7 attack,
1d10+4 damage,
2 attacks per round, 50% hit chance
Aberration:
+7 attack
1d8+7 damage
2 attacks per round, 50% hit chance
that comes down to 420(heh) damage per day
with 20 cha;
EB:
+8 attack,
1d10+5 damage,
2 attacks per round, 55% hit chance
Aberration:
+5 attack
1d8+7 damage
2 attacks per round, 55% hit chance
that comes down to 484 damage per day
so we get a difference of 64 damage per day or 15,2%. Little less extreme in RELATIVE terms than low level greatsword, but it's still a lot.
especially if it is 15% in your MAIN job, or just the job that you like the most while playing DND(or combat part of DND)
Or just to turn it around, if you are thinking that it is so irrelevant, why do you bother if players take it as an option or not?