Once you’ve decided “I want to give them a scroll now” or “I think it’s time they deserve to get a spell staff”, how do you go about choosing which spells those are?
I assume different people have different ideas here, there isn’t one right answer. This is meant to be an open-ended discussion prompt.
I like to use the weird spells that players rarely take due to being less than optimal. That way they still get to use them situationally, without having to “waste” a slot.
The exception is when there’s a particular story reason for a certain type of item to crop up; for instance, a group of assassins hunting a powerful mage might have a lot of dispel and antimagic type things.