vbank layering doesn't make sense to me

[SRB2er]

So from what I understand, vbank allows you to swap to a separate palette (and back) on the same frame, allowing for 31 colors (or 32 if you don't want transpareny) per scanline.
..but then it also says this:

The banks are rendered to the screen simultaneously with any opaque portions of bank 1 appearing on "top" of bank 0. Bank 0 data is rendered using the bank 0 palette and bank 1 is rendered using the bank 1 palette. OVR TRANSPARENCY (0x03FF8) controls which palette index of bank 1 is considered transparent. Currently this behavior cannot be modified.

This part...confuses me.
If you want to draw on top of something, draw it after.
I don't understand what makes this part special about vbank(1)
Can someone explain it better?

[Weeppiko]

vbank doesn't change a palette, it swaps the layer that does have it's own palette: vbank(0) is the lower layer and vbank(1) is the upper layer. Everything drawn on vbank(1) will be on top of vbank(0), and since it is on top, one of the colors need to be treated as transparency.

[Weeppiko]

One example of vbank usage would be using scanline effects only in one of the layers, like this (background on vbank(0) and sprites on vbank(1)):