Honestly, it has more to do with revealing their governing style than about their ability to give you those promises. It tells you who they are and what they want. And the way they talk about it reveals something of how they think and how they'll be able to deal with other leaders (in and out of the country).
We know they'll be able to give you at most one or two, but it's not really their job anyway. Most of those promises are the jobs of legislators, not executives. But people imagine that they're learning how that person will execute, a thing that's a little hard to convey otherwise.
https://www.foxnews.com/politics/trump-signs-executive-order...
At this point, I don't even know why presidents promise anything... we only have a choice between 2 of them...