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.
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.