SHA256 is a hashing algorithm, which is supposed to be able to create a checksum fast. However, you don't want your passwords to be cracked at a fast speed, so

SHA256 is a hashing algorithm, which is supposed to be able to create a checksum fast. However, you don't want your passwords to be cracked at a fast speed, so you use a KDF like bcrypt or scrypt, which is much slower and harder to implement on GPUs.