To clarify an important distinction I'm not sure is clear though... Not existing equipment, a purpose built type of equipment (ie like final fantasy's long preserved weirdly translated meaningless "bangle" than hybrid things like armor/bracers/rings/etc granting attunement points).
There are a couple high level reasons that really provide a lot of benefits & I've talked about a few of them already. A while back (months? years?) crawford said something like "what we were surprised to find out is that some groups don't like to blaze through those early levels & are happy just taking their time to develop things.. we did not design for that", I think that he might have also mentioned that something in the book ( he was playing up at the time (vrgtr?) was intended to help those groups. There's also wotc's obvious desire to have it both ways with magic items are core/not required/always a boon. That's 4 different groupings of playstyle plus shades in-between & they may or may not overlap but all of them are serviced in ways they need by not using a prof bonus or level linked value.
- Shifting the ceiling for "attunement points" from being linked to an unstoppable one size fits all thing like prof bonus to being based on something equipment based allows for the individual GMs of all those groups above to cleanly tune the level of magic items that are right for the group & GM's game/campaign without ugly hacks & rebuilds or being stuck with a decision made early on before the group could see what was right for this campaign.
- The GM is able to maintain or raise the ceiling as needed regardless of leveling speed & could hypothetically do so using a light touch that doesn't require nerfs or forced rebuilds on a PC by PC basis if there is a wide disparity in CharOp leading to a frustrating spread of build power. There is an added benefit where a very subpar PC who suddenly goes from needing the GM's help to being very heavily optimized in ways nobody expected the GM has avoided the traditional sort of problems that a single powerful magic item would have caused & the help once given can phase out on its ownsimply by not lifting that one PC's capacity items when/as much after they lift everyone else's.
- The GM is able to use magic items for incentive more freely knowing that players might need to choose between a hypothetical scenario* like +1 weapon & +1 armor or holy avenger & off the rack armor and then insert those ceiling height bumps at points that make sense in the campaign by choosing when they award improved attunement point items rather than merely having PCs accumulate the experience or enough consecutive sessions to advance in level. That one is especially useful if the players are faffing about exploring the world or having fun killing time with sidequests or filler adventures & by extension digging into worldbuilding rather than focusing on what might often be called a main questline that might be severely disrupted & need significant retuning from the GM if PCs are suddenly far more powerful thanks to what a prof bonus linked point bump allows.
- the ceiling can be adjusted to accommodate low medium & high magic campaigns in tandem with the quality/power of magic items the GM chooses to give out rather than the GM needing to do it through supply alone.
- etc
* It's an example intended to be as clear as possible, not something built considering balance or whatever
