Instagram re-encodes everything you upload. Sending the exact canvas it wants (9:16 for Reels and Stories, 4:5 for feed) means its encoder only compresses your video, instead of also cropping or padding it its own way.
Crop-to-fill keeps the center and trims the edges, which flatters footage whose subject sits mid-frame. If the action lives at the very edge of a 16:9 shot, that edge is what gets traded for a full-screen 9:16.
Mind the UI: on Reels, the caption, username and action buttons overlay the bottom and right of the screen. Keep text and faces near the middle of the frame and nothing important gets covered.