Well that's mostly because it is a all in one item for junglers. It gives you some AD to kill the jungle creeps faster, some armor so you could jungle for a longer time and some lifesteal that let's you jungle even more. Also the chance on hit effect makes you jungle as a boss if you're lucky. But the really important thing is the free ward every 3 minutes. You'll probably want to put it first than the normal wards because of simple logic that it recharge faster then.
The ward can save you up a lot of gold if you're buying wards usually which you should be doing if you're the jungler. It saves up 75 gold every three minutes, if you put it as fast as the cooldown goes off. To sum up, you'll want to get it with most(if not all) of the AD junglers.(Warwick, Ganglank, Nocturne, Shaco, Lee Sin, Xin Zhao, Olaf, Irelia, Jarvan, Yi, Nasus, Udyr or anyone I forgot to mention.

Wriggle's Lantern is also a nice item for some champions while lanning. Why? Well it's obvious, it let's you farm faster if you need to clear the minions fast and not to last hit, it gives you some nice lifesteal so you don't have to back that often. Also some armor and AD is always nice, isn't it? Of course let's not forget the free ward that let's you ward that bush or dragon every 3 minutes without backing. I personally like it most on Ezreal, because it can scale off his Q. Also it's a viable item for all the ranged carries, but it's a rare pick, because they can just get some pure damage. I think it's good if it's bought early and helps lane a lot.

Pros:

A free ward every 3 minutes.
The chance on hit makes you jungle faster.
The lifesteal let's you jungle/lane longer.
If you're lanning, it let's you farm like a boss.

Cons:

Scales poorly into late game, so you'll have to change it into something else.
Is usable only by AD junglers.
Is usable by only AD lanners.

So Wriggle's can also be used in some fun ways that you may not know.
The best one is viable when you play Lee Sin. You just need to put down a ward and charge into it with your W spell. And bravo - you've escaped or catched up when you could've used flash or just simply failed.
The same trick as mentioned above works with kat, but she shouldn't get it. BUT you can save your kat's life (not the meowing one's) by letting her SHUNPO on it.
Also there is a little trick that's really hard to use and I learned about it from a friend. When someone is chasing you can put a ward right in front of then. It makes then bump into it an run around it until it becomes invisible.
Don't forget that these tricks also work with simple wards so use them.
And lastly here's a map of the best warding spots on summoner's rift.(sorry Akeii who's map I put here because I'm too lazy to make my own one.And the TT map. Also took one from google pics.http://na.leagueoflegends.com/board/showthread.php?t=237514 ( A detailed warding guide)

Well long story short, Wriggle's Lantern is probably the best AD lane farming/jungling item in the game. Getting it may save you some gold for the wards and make your jungling stronger. Just don't forget to use the ward as some players do. And remember - get it early. So soldier now that you know how to use it, go and make me proud!