The primary purpose of your margin settings is to create a space around the outside of your screenshots, allowing your screenshots some room to breathe and ensuring they do not suffocate (or cram-together) the text or other images where your screenshots will be used.
Use the margin settings to adjust the width of the space around your screenshots to best suit your needs, and add a background color that matches your brand guidelines. You are also able to set a fully transparent background to allow a setting where your screenshots can be used, for example, on a help center that supports both light and dark modes to blend the image perfectly on both. These margins will be applied consistently across all of your screenshots.