It's not about saving money, but not having to rely on a third party.
And also every important, it's about power efficiency, where Apple excels.
There are also rumors they plan to integrate it in the SoC.
It’s can be both and all three.
As for integration, if Apple is doing the development and TSMC is fabricating them, it’s a way to stay ahead. Or catch up to the pieces sold by others.
Either way.