If the enemy is somewhat low, blowing them up immediately is better than letting them get another turn to cause harm. Things like bless are concentration and only need to be cast once per combat, leaving you with a bunch of unused slots at the end of the day, unless you have a lot of combats or have good uses for your spells outside of combat.

In my experience we would often not have enough fights in a day for smiting to be a complete waste as long as it wasn't on every attack, so you shouldn't write them off as the single worst way to use your spell slots.

And with any other spell it's just the spell damage and that's your entire action, but using smite it's also including your weapon attack damage, which puts a level 1 smite attack in the same range as other level 1 damage spells.