I've always taken these Smart Replies as equivalent to Facebook Messenger's "button that sends a big thumbs-up emoji to acknowledge something." (It replaces the Send button when you don't have anything in the input box.)
I.e., these responses are there to serve as various textual forms of "I acknowledge that I received your message and am hereby discharging your social expectation that I will reply to it, by doing so with an information-free message."
I assume that Gmail would just suggest using an emoji rather than text, except for the fact that 1. etiquette says that an emoji is too "casual" for office work, but a one-word answer is just fine and exemplary in its professionalism; and 2. there are devices still in use that can receive email but are old enough that they can't display emoji, so textual equivalents are a better lowest-common-denominator.
I.e., these responses are there to serve as various textual forms of "I acknowledge that I received your message and am hereby discharging your social expectation that I will reply to it, by doing so with an information-free message."
I assume that Gmail would just suggest using an emoji rather than text, except for the fact that 1. etiquette says that an emoji is too "casual" for office work, but a one-word answer is just fine and exemplary in its professionalism; and 2. there are devices still in use that can receive email but are old enough that they can't display emoji, so textual equivalents are a better lowest-common-denominator.