(Math, Personal opinion and bad english incomming)
If you only have 5% chances to hit, a +1 boost make you succeed 100% more often but on a single dice roll it has only 5% chances to change the outcome (10% if critical fail matters)
If you have 50% chances to hit, a +1 boost make you succeed 10% more often and critical hit 100% more often but on a single dice roll it has only 10% to change the outcome.
Every permanent +1 boost feel extremely valuable in the long term (that's why the 18 in primary stat at creation is mandatory) because you can see the statistical increase in success rate.
Every temporary, situationnal, or only for one roll +1 boost feel useless : a hypothetical +1 to attack for a full 3-attack turn with 65% to hit the first attack will not change the outcome 81% of the time.
