There are some Huawei Y3 ( CRO-L22 ) mobiles that have strict restrictions on unauthorized flashing by the company. They require a secure handshake during flashing. DA Auth File removes these restrictions and allows SP Tool to establish a verified connection with the mobile. Until this handshake is completed, flashing is not possible. This file in a way takes the password from the mobile's motherboard and gives it to the flashing tool, so you can modify the phone.
If you have ever tried to flash the userdata partition of a Huawei Y3 ( CRO-L22 ) mobile, you will know that this partition usually stays in secure mode. When a flashing tool tries to modify it without the DA file, a failure message appears. The DA file provides encryption keys to the tools so that they can securely access sensitive areas like userdata, metadata, nvram, etc. Especially when you need a complete wipe before selling the phone.