Browser vendors use ASN.1 encoding for RSA certificates in the TLS protocol. RSA public keys are just a number, so why do we need encode them to something else? That increases the risk of security problems.
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Think about this, say my public key is
What does that mean? Big or little endian? Hex or decimal? Specifically for RSA, where is $e$, where is $N$? What is this public key authorized to do (encrypt, sign, etc)? Who has signed this key? To whom is the key linked?
While a public RSA key is "just a number" there are a lot of reasons to encode the key in a standard way so that every computer everywhere knows how to understand it, what it is authorized to do, who it belongs to, etc.