So the first way is for taking advantage of how some chords just can’t exist in certain keys using plain I-II-III-IV-V-VI-VII chords.
For example, in C major, you can’t have any chords with an accidental as the root note. Let’s say I want to put in C# major; by default that’s impossible because its root note is not in the C major scale.
So, I need to switch modes (using the Relative setting) to change the root note of the scale.

We need to change scales so that we can find a mode that, when I borrow from a different mode, will have C# in its scale at a spot where it can be the root of a major chord.
In major keys, the major chords are I-IV-V. So, we need to find some scale that has C# on scale degree 1, 4, or 5. If you try this, you’ll find there is no such combination, so you need to pick a different mode.
It takes some searching to look for the correct combination of relative mode and borrowed, mode, but it is possible to get chords of any scale degree of any quality (without extensions) in this way (though it may not be analytically correct in the context of the song).
For our C#-in-C-major example, there are a few ways of getting there: C-Major borrowing the bII of C-Phrygian (as Db), and C-Major borrowing the bII of C-Locrian (as Db), and B-Locrian borrowing the #II of B-Lydian (as C#).
You can also use the Search function, Tone Sets, and Bass Sets to get the chord you need as well.