The main reason you'd want to pass on a boon is, once adventure 3 has begun, to permanently purge those weaker cards from your possible future treasure. Removing Basic and Elite cards from the game means you are more likely to pull better stuff. The option to "autofail" boon acquisition rolls has some other niche applications as well - imagine, for example, a villain that penalizes you for having, say, spells in your hand. A "fighter" type character may not want to roll to even try and gain a spell that does them no good, and may in fact hurt them later in that case. That example was made up but such things are not unheard of in the tabletop version of the game.