Not all covers ruin songs. Some are the same thing, no better and no worse. Some are pretty much the same thing, but with a band you might like better, or a band you might like less. Some are drastically different takes. Some of the most respected songs are, themselves, covers. For example, did you know that Aretha Franklin’s Respect is actually a cover of Otis Redding’s song from a decade before, with a distinct take on the song. “The Lion Sleeps Tonight” was covered by many, many groups after it’s original creation in 1939 before The Tokens made it the song everyone knows today (and then Disney made it the song actually everyone knows). Goldfinger has it’s own rather successful take on 99 Luftballons. “Singing in the Rain” was sung two decades before Gene Kelly did his version.
Cover songs are also a great way for audiences to get to know new artists. They may not like your particular lyrics, or sounds, but if you can just prove to them that you do have some musical talent, then they’ll give you some leeway.
Songs stand outside the original performer. They’re just “the song”, not “so-and-so’s song”.