@Mark_H Good question, this kinda question is a big step forward in your musical journey.
Why? Because you are wondering why the heck guitarists do what they do (or how musicians make sense of what they hear & see). Blue's & Guido's answers are worth re-reading.
If you wondering, but why, why why are there so many ways to play the same thing? Well...frikkin' vocalists and other limited instruments (e.g. Horns)! While guitar/piano can (fairly easily) play in multiple keys, vocalists and many orchestral instruments tend to have a range of notes they can sing/play (we are not all Mariah Carey..thankfully!) so we humble guitarists need to be able to accommodate them... (This is all said with tongue in cheek ;p)
Maybe try learning on guitar something you can hum/whistle/sing without thinking. For me, it's nursery rhymes (Happy Birthday, Row yer boat, Twinkle Twinkle, Somewhere over the rainbow, Old McDonald, etc...). It's that first step of ear training, but also gets you to try out playing with something you can vocalise and see how it feels/works.
I'm happiest playing Row yer boat in E, but a octave up from the low e on guiatr - there it feels 'right' for my crappy vocals. But Old' McDonald I play in C - feels right there. And somewhere over the rainbow feels right for me in G. Happy birthday I do in D - though that's not really most peep's preferred key, works for me.
While there are right n wrongs for covers - there's (like mentioned) a lot of ways to skin the cat. Learning them all is (unbelievably) half the fun!