[SunnyDee]

I think a lot of the difference in the answers you've gotten comes down to learning styles. Some of us (me, for sure) learn better if we understand intellectually what we're doing and why. Others don't want to intellectualize it and they just do it, learning songs, from bandmates... etc. We also, I think, have a third group who've been playing for decades and, maybe, as they say, they just played a lot of songs before sensing theory or learning it later, but they had decades to do that or maybe a great ear. Personally, I didn't have decades or a great ear or musical parents or anything, and I doubt I'd have ever just sensed it, because I started at zero and had never sensed it the whole rest of my life hearing music, so... yeah. I've been playing for about three years and learned theory and ear training and history and other stuff at the same time. You say you "should" learn. I don't think there's a "should" to it, it's just a matter of whether it'll be useful to you or not.

So, what's it useful for...?

Well, it's the difference, for example, between remembering that you'll need to put a capo on the 5th fret for song X so it sounds like the record when you play the open E-chord shapes you learned - the shapes you learned without knowing why they have those shapes...

or, instead...

knowing that you could also play song X without a capo using open A-chord shapes and knowing how that's going to be the same key but change the voicing a bit and knowing what that means and why that is, so you have more choices in how you want to make X sound.

It's the difference between knowing where to put your fingers to make a certain sound and knowing how many other places you could choose to play that sound.

It's knowing you could play in any key with or without a capo on a guitar strung standard or in an alternate tuning or even on a different instrument entirely. Not that you could physically play it instantly because that's technique and practice or some crazy amount of god-given talent, but you'd understand how to instantly, because that's what knowing theory is...

It's knowing how the music all fits together, how it makes these patterns of sounds that we find pleasing or interesting. And that's an amazing tool that opens up whole worlds, including the one that lets you talk to a lot of cool musicians.

There's a lot of inefficient or awkward teaching of theory in the guitar world, so, if you do want to learn, question the methods you see. People will say you have to memorize all the note names and the circle of 5ths, but this is not the most efficient way. They will also say you should start with piano and learn to read a score... but these are both traditional, classical pedagogical traditions that were originally aimed at creating classical musicians, people whose job was to recreate the score... but, this is not usually what adult learners are doing on guitar, so question that idea, too.

In the end, if you decide you just want to know where to put your fingers, and, like many, practice the pentatonic scale a lot until you can do it really fast, and you're happy with that, then that's fine, but if, while moving your fingers really fast, you find you really want to know why you're playing the pentatonic, and why it's the minor one most often, and what do we use the major one for, and what is minor and major, anyway, and why do these "5 patterns" really seem like one pattern moved around, and, by the way, why are these chord shapes similar all over the neck...? Then you want to learn some theory.