The selection button works because it asks almost nothing from the client. They already know what it means. They see a photo they like, they tap. No instructions, no spreadsheet, no "send me the file names when you are done."
It matches natural browsing
Clients do not usually review photos like editors. They react emotionally. Selections let them capture that reaction immediately.
It reduces message chaos
Without selections, selections become screenshots, circles, voice notes, and "the third picture after the one where I was laughing." The selection button keeps the decision attached to the image.
It gives the photographer better data
A selection list tells you what the client values. That can shape album choices, retouching priorities, and even what you show more of in future shoots.
Make it part of your message
Tell clients: "Tap the check icon on the photos you choose." That one sentence can save days of confusion.