Well for one, it can be used for practically anything, making it a good catch-all ability similar to heal, fortify, revive, and clairvoyance. It helps you max out last hits and increases your lane pressure, especially when you're losing on the lane and the turret is trying to steal gold. You can grab jungles-of-opportunity a few levels earlier than without it on the same champion (I often snag a super-fast mini-golem on my way back into a side lane when we have no jungler), you can grab buffs faster, you can steal enemy-side buffs more quickly, and it helps for bringing down dragon and nashor faster - critical since the time it takes to bring them down is the central downside to getting them. These are things that every character in the entire game appreciates at all times. Plus the cooldown is boss, and the mastery is at the top row. Perhaps importantly, it's reliable as hell since it gets the full benefit the instant it is cast, and it's never mitigated, reduced, or blocked unless Karma is putting shields on tank minions or something like that; every single cast gets you exactly what you wanted, which is rare in a PvP game.
In particular regarding last hits, the idea that you can reliably last hit everything even WITH Smite is complete fallacy and doesn't actually happen in a real game - ESPECIALLY if you're camping the turret and giving the enemy free ganking time. There's a wide band of players in the "decent" range and below "genuinely good" where lots and lots of them imagine in their minds that if they just optimize last hits, then camping the turret is the best strategy because the enemy won't get fed. Sorry, this game isn't so poorly designed. It's great if it stops you from feeding, but lane pressure gets more minion feeding - risk vs. reward, like a real game should be. At my rank, people still give me crap about lane Smite, but players almost never camp the turret unless forced by the enemy; keeping up lane pressure is not a very disputed strategy at higher levels of play. All this is not directly related to the question, but if you operate under this misconception, you'll be particularly incapable of appreciating what Smite in a lane can do, and so I thought I should explicitly point it out.
All this alone wouldn't make it worth taking into the lane with most characters, but there's another 10-15 minutes of explanation for that. But you didn't ask what makes it worth it, only what makes it helpful (implying, "what makes this so much better than no spell at all"), so I figured I should start at the absolute basics. In particular, it's helpful because you don't need to win fights, you need to get gold and xp. Warwick doesn't need Smite to jungle - he'll still win there - but most players take it because it makes him more efficient. The idea that smite can't help in a lane is tied to the idea that you'll always "win" against a minion wave, so why bother? Well you need the gold and pressure too.
Oh yeah, another thing people ignore is on-kill effects. I always LOL extra hard when I get crap for Smite on someone like Alistar, since in addition to the usual benefits, smiting a minion to make my Triumphant Roar cool down sooner even helps in champion fights. Cho gets health and mana for it, and any and all spell vamp in the game gets a nice chunk of health when Smite is cast. But yeah I won't bother going into the rune/mast/champ reasons for why I do it. Just pointing out that there's a lot of interplay between mechanics in this game that people tend to ignore.
