Boombro and Eduardo pretty much covered all the reasons I'd say, but I wanted to throw my peso into this mix anyway
Technically, your question would be better answered if we knew your overall playing style. You see, the question isn't what makes followers special because that would be the question about anything in any game. The real question is what would make it special to you?
If your overall goal/preferred playing style is a certain way, advanced or not, a follower isn't going to be special... to you, but like anything where there is a variety of options, there is a good chance that there is a follower or two you would enjoy.
While this no means is fully accurate but just provides examples of what I mean, say you like...
... a challenging Skyrim but some mods that are interesting would jack your Skyrim from being easy to insanely hard - Find a strong follower, maybe even an OP Follower.
...an immersive Skyrim that you picture to be like some sort of fantasy adventure to immerse yourself in the role of dragonborn and the world of Skyrim rather than just a game to plow through - Find a companion that actually feels like a companion, such as Vilja and Sofia (there are plenty more btw (and no, the simple ones that are just a face do not have more endorsements in general and the two highest endorsed followers are complex to say the least)).
...You like your Skyrim to be beautiful in every way from landscape to people (and like the landscape you don't really need the people to talk all the time) - Find followers where they put a tremendous amount of effort into looks rather than functionality and dialogue.
Now this isn't to say that there aren't followers that cover multiple categories at the same time. This was just a quick summary/examples of why in order for a follower to become/be special to you, they must fit to your game.
Decide on the kind of play through you wish to have and THEN look for followers because with as many followers that are available, there is likely to always be a handful that would work for you (especially with additional mods) regardless of your playing style.
In short, there's somebody for everybody... usually, lol.
EDIT: Had to clean that up a bit.