ReelPile never asks you to sign in to Instagram, and cannot. There is no login window, no cookie file, no account of any kind. Reels are fetched the way a visitor fetches them.
Earlier versions did offer a sign-in, and it was a bad deal for you. It meant the app worked under your credentials, so anything Instagram does about automated access — throttling, a checkpoint, in the worst case a suspended account — landed on the person who bought the app. A website that downloads for you carries that risk on its own servers; a program on your Mac would have been handing it to you.
It was there because the code assumed anonymous access stopped working after a handful of clips. That was measured before removing it, on fourteen real reels: all fourteen downloaded, none were throttled, and thirteen arrived at 1080×1920 — the largest size Instagram offers. The sign-in bought neither reliability nor quality.
If you had signed in on an older version, that saved session is deleted the first time this version starts. Anonymity that is only true in the code and not on disk is not anonymity.
What this costs you: nothing that has been measured. What it could cost you one day: if Instagram ever tightens what it gives visitors, the fix would be an update to ReelPile rather than something you can do yourself.
Something here wrong, or something missing? Tell us — the manual is corrected against the app, not the other way round.