Howdy all! Sorry for the necro-post, but after tonight's game session where my players pointed this out this magic item descriptor to me, I decided I needed to do a little research to figure out what the heck is going on. My players were taking the implication of the wording literally, thinking that after 24 hours the conveyed bonuses became truly permanent even if the magic item were later removed. To that I sternly disagreed, but lacked the canon explanation as to why.
That led me to Google in search of an answer, and ultimately here to this discussion thread.
Having read through this one a few times, and gone over the cited rule above straight from pgs. 554 & 555 of the Core Rulebook, it finally donned on me what the purpose of this common descriptor actually is.
Whenever a magic item says it's bonuses are treated as "Temporary" for the first 24 hours, it's actually just speaking towards the timing of the bonuses' effectiveness. Generally speaking, any magic item or spell that bestowes a long-lasting bonus falls under the "Permanent" category by definition of its prolonged duration of effect (even if the word Permanent is a misnomer since few things are genuinely permanent). But the problem with that is Piazo also set the rule that Permanent bonuses take a full 24 hours to gradually go into effect. By having various magic items initially bestow their bonuses on a 24-hour "Temporary" basis, they get around their own rule here.
So, simply put, the reason why magic items like the Belt of Physical Perfection and such include the line, "temporary for the first 24 hours", is so the character can get instant benefit to the item's bonuses without having to wait a whole day. That's it! There may be other benefits as noted above, but its just Piazo's roundabout way of saying, "Put it on and use it immediatelyl, not this time tomorrow."