Prestige Class Dreamer - What does "Dream Spells" do?

[Dominion]

I am following "Ultimate Prestige Classes Vol.1" here which says the following: https://prnt.sc/1033v0r

I know how to collect dust etc., but I am wondering why I would use this, seeing as I cast the spell normally except it is treated as a phantasm which allows targets to fully negate its effects through a will save.

That's... a drawback and no advantage. At least as far as I understand it.
I don't see why I would do that.

Or does the first part mean I can cast a spell on a higher level without using a higher slot?

[Vaern]

I am following "Ultimate Prestige Classes Vol.1" here which says the following: https://prnt.sc/1033v0r

I know how to collect dust etc., but I am wondering why I would use this, seeing as I cast the spell normally except it is treated as a phantasm which allows targets to fully negate its effects through a will save.

That's... a drawback and no advantage. At least as far as I understand it.
I don't see why I would do that.

Or does the first part mean I can cast a spell on a higher level without using a higher slot?
It's hard to tell with just a screenshot of the one ability and no context as to what the class actually is, what it's meant to do, and how it's meant to work. At a glance, it kind of looks like it would allow wizards or clerics to expend a prepared spell slot to instead cast any spell they know of the appropriate level, as it says that the spell being cast must be known by the caster and is treated as though it is being cast spontaneously. But without seeing the entire class description it's impossible to know whether the prerequisites even allow a prepared caster to take the class or if it requires being able to spontaneously cast spells of a certain level.

In the case that the class is specifically restricted to spontaneous casters, it also looks like the ability may allow you to use dream dust to cast spells rather than expending a spell slot to do so. The potency of the dose may determine the effective spell slot that you are emulating. Even as a phantasm whose effects are completely mitigated on a successful save, this can still be very useful for casting spells on yourself and allies like invisibility or fly as you can simply forego the saving throw to willingly accept the effect. The benefit in this case, of course, is that you can hoard dream dust to have a ridiculous stockpile of bonus spell slots at your disposal at any given time.

[InvisibleBison]

I think it lets you cast a spell using dream dust instead of a spell slot.

[Dominion]

Okay thanks :)