This question already has an answer here:
Pretty much what it says in the title. I recently asked this question and it has been suggested that it is unanswerable.
A previous question of this type is here, which by my understanding was decided to be a legitimate question for the site, but I could be wrong.
Update: Based on the answers I've gotten here, I've updated the question to make it clear that idle speculation isn't helpful.
Update 2: Based on doppelgreener's excellent breakdown in comments of the issues with my question, I have reframed it to be about design intent specifically.
The comments were deleted, but here they are for posterity:
I'm voting to close this question as too broad. At the moment, it is inviting speculation on three different fronts: (a) what various benefits are there to having a stat of 19 rather than 18 (regardless of whether an item does it), (b) what benefit is there to having an item that would do that, and (c) why did the designers do it that way? (a) and (b) aren't necessarily related (as observed by JasonK's answer), and neither necessarily answer (c) which could be, for instance, a designer quote saying "I like odd numbers, they're cool." – doppelgreener 11 mins ago
I don't necessarily have a problem with (a) and (b) being wrapped up together -- except whilst (a) is objectively answerable, (b) involves opinion and speculation. The stake-to-the-chest for this question though is that neither of those will answer why the developers did it. – doppelgreener 6 mins ago