Question

Why do some block cipher modes of operation only use encryption while others use both encryption and decryption

Answer 1

it is used to generate something to XOR with the plaintext.

Step-by-step explanation:

Some mode of operation only use an encryption function because it is used to generate something to XOR with the plaintext. There is no point decrypt the generated bytes. To decrypt the ciphertext, you just need the same stream of bytes.