"position: fixed" removes the element from the layout and positions it relative to the window. You can scroll the page any which way and the element will stay where it is on the screen, making it a good choice to handle X/Y positioning of the modal.

I asked how it helps with z-indexes not XY?

Also drop downs are "attached" to some element in the UI, and you probably want those menus to scroll with the page

"position: fixed" puts an element in a new stacking context, freeing it from the z-index of its parents.

The person you were responding to said they use "position: fixed" so they don't have to worry about z-indexes, and I was explaining why that works. See sibling reply for the more technical explanation.