What one live view answers
A reply spreadsheet is a snapshot that starts going stale the moment you save it. The messages land in three different places, you copy them across when you remember, and the file is never quite current. Meanwhile the real question, who is coming, still has to be answered, and now it lives in a document no one trusts.
An RSVP tracker is the opposite: a live list that fills itself in. Each guest who replies moves their own name, so you are always looking at the current answer rather than one you transcribed last week. No copying, no second file, no "did someone already say yes in the group chat?".