When do I need the cassette low spacer

You don’t say what size the cassette is but presumably it is a wide range model.

11 speed compatible freehub bodies are 1.8mm longer than 8-10 speed ones. Some wide range 11 speed cassettes can fit on a 10 speed freehub because the the large diameter inner sprockets can overhang the inner end of the freehub body a little and clear the drive side flange and spokes. Such cassettes come with a 1.8mm spacer for installation on a 11 speed freehub.