I am not talking about password compromise, but if somebody gets my
phone, he just needs to turn it on to get data decrypted, so what’s
the point? For what cases is this encryption designed?
I'm not sure from where you are quoting this statement, but it's false. Physical access to the device does not guarantee automatic decryption of the disk.
From the official Android documentation:
Upon first boot, the device creates a randomly generated 128-bit
master key and then hashes it with a default password and stored salt.
The default password is: "default_password" However, the resultant
hash is also signed through a TEE (such as TrustZone), which uses a
hash of the signature to encrypt the master key.
When the user sets the PIN/pass or password on the device, only the
128-bit key is re-encrypted and stored. (ie. user PIN/pass/pattern
changes do NOT cause re-encryption of userdata.) Note that managed
device may be subject to PIN, pattern, or password restrictions.
In the comments I've also referred to this question that answer a similar (if not equal) question.