Single Dice Resolution Systems Suck

A +1 boost in  a d20 system is a teeny 5% boost to your chances of success. That is because a single dice produces flat results.

Flatter than an A-Cup Angst Anime Character

Every possible roll on a d20 is equally likely. Systems attempt to account for this by moving the goalposts of what you need to roll over to succeed, accounting for bonuses to calculate the chance of success based off of the bonus you have.

After all, a +10 bonus guarantees you will hit a goal of 20 half the time.

3d6 resolution is different.

Curves like the girl who doesn't reciprocate my feelings

See that bulge in the middle? The tendency for results near the middle of the range of possible outcomes to be more frequent? That right there is awesome. It's still random, yes, but now skill bonuses matter- and become more interesting.

An effective skill level of 10 in GURPS represents a 50.0% chance of success (p.171 is amazing). A +1 raises that to 62.5% chance of success- an increase of 12.5%. That's two and a half times more bang for your +1 buck vs. a d20 system.

Now, the further you move away from the middle, the less the increases in skill matter. It's not until skill level 14 at the top of the scale that a +1 bonus in GURPS is less powerful than the same +1 bonus in a d20 system game.

The change in percents as your reach extremes is great- if you're extremely low-skill for some reason, jumping a level up is a tangible boost to success- while bonuses at the top have less and less effect, eventually becoming moot (17 and 18 always fail). In GURPS this frees you up for things like harder combat maneuvers, techniques, dangerous shit... in d20 it uh... why would you ever stop accepting bonuses to do rad stuff? You're losing 5% each time, which is trivial, sure, but you're still shooting to get above the goalpost, where in GURPS you can stop worrying once your skill hits 16 most of the time.

The goalposts you're aiming to roll under in GURPS vs rolling over in d20 aren't nearly as arbitrary. There's not a great deal of math involved figuring them out- just looking at the bell curve distribution anchors the resolution mechanic to a norm- you can expect a lot of 8-12's being rolled.

Special shout-out to 11d10-10 as producing a very beautiful 1-100 bellcurve distribution of roll values.