What magic item benefits are built into the expectation of the game?

I often read that magic item progression is built into the game’s difficulty, and that it’s important for PCs to keep up with the progression in order to not fall behind.

Exactly what progression is “built in” and “necessary” for a PC?

For example, I usually buy an anklet of translocation for most of my PCs around level 5, and I also usually buy a ability-boosting item around level 6. I imagine the game anticipates that ability boost, but it probably doesn’t anticipate the anklet.

Similarly, most players I know like to buy a handy haversack at some point between levels 6 and 12. I don’t imagine the game designers built that into the CR of the monsters.

The Book of Exalted Deeds, on pg 31, provides a chart of progression for characters who take a vow of poverty. Is this about what the game designers had in mind for magic item progression? (And if so, how do you account for the bonus feats?)

Another way of asking this same question is this: there is a published Wealth by Level chart. How much of that gold is sort-of allocated to necessary magic items (resistances, weapon enhancements, ability boosts, etc.) and how much of it is allocated to strange wondrous items don’t directly influence the PC’s numeric values? (like an anklet of translocation, a wand of silent image, a handy haversack, etc.)