I think for the upvoter, sending an upvote can trigger that same feel-good, "I agree to this!" dopamine hit as an emoji reaction would.

For the upvoted, seing their magical internet points go up can also feel the same as the notification that they received your heart emoji.

BUT - for the network, having scores be hidden and not displaying a loot of emoji reactions on each post makes a huge difference - it changes the incentives.

You don't get socially rewarded for engagement, because the loot is invisible in the thread (aside from being shown higher on the page). This disincentivizes Reddit-style snark, Twitter-style dunks, etc.

This probably disproportionally tends to attract the kind of people who like discreet positive reinforcement for smart well thought-out comments.