Why are the right halves kept unchanged in Feistel networks? As I'm going through the Feistel network in the DES algorithm.

While encryption uses a Feistel Network, the input block is divided into two halves left ($L$) and right half ($R$). In each round, the right half $R$ goes through unchanged. But the left part $L$ goes through an operation $F$.

I didn't understand why right halves is kept unchanged in a Feistel Network. Can anyone explain?
This structure allow us to make the process invertible. A Feistel structure creates a pseudo random permutation from a pseudo random function.
We want someone with the key to be able to calculate it in reverse and get the plain text from the cipher text.

Also recall that the after two rounds both sides get scrambled and after many rounds they are thoroughly scrambled to make deciphering without the secret key rather difficult. DES is no longer considered secure but the Feistel structure is still used in other block ciphers.