In laymans terms, its the difference between having a guard, and a locked vault.
When you have only a lockscreen, its just like having a human guard that will ask you for a password/PIN. If this can be circumvented or bypassed - either via a vulnerability - which would be equvalient with social engiinering the guard into giving you access without knowing the password/PIN, or if the lockscreen can be forcefully removed, would be equvalient with killing the guard, access is subsuquently granted.
If you instead have the device encrypted, its like a locked vault. No matter who you kill or bypass, you can't get into the vault without the correct code or key.
Using device-encryption while off, along with screen lock while its on, is more equvalient with the bank vault being closed and locked during closing hours, and a human guard during opening hours. The guard is more convient during daytime since you dont have to open and close the heavy vault door all the time.
The reason you can't use fingerprint authentication with encryption, is because a encryption key cannot be derived from a fingerprint. Instead, you have to have some hardware that performs an authentication - like the guard - and releases the key upon correct authentication.
A fingerprint authentication isn't like providing a password, since everytime you place the finger, you are not placing it exactly like you did when enrolling. Instead, a very complicated mathematical calculation needs to be made, to judge if the fingerprint is enough similiar to the enrolled fingerprint, to be accepted.
Imagine like taking a photo of your house. Next day you see a house and want to know if its yours. You take a new photo. If you would hash these photos or compare them bitwise, you would always come out to a false decision, because the photos will be slightly different lightning and angles.
Thats why you cannot derive an encryption key out of a biometric authentication.