Lock down will do things like disable biometrics, but the phone has still been unlocked and is no longer in a fully encrypted state like it is if powered off.

Rebooting your phone actually encrypts the device, and it will stay encrypted until the first unlock. But the side button simply disables biometrics. The phone is still unencrypted on the backend, so other workarounds (like some sort of exploit that allows them to bypass the PIN) will allow them to see the phone’s contents.

This is particularly important because cops can image a phone after they seize it, to try and hack later. If they manage to image the unencrypted phone, that whole process is much easier. But if the phone is encrypted when they image it, they’ll basically just get white noise.

It’s been a while since I had a Samsung, but iirc there’s an option to add it to your power menu. 

More importantly, rebooting or powering off the phone is more secure anyways. You should just do that in 99% of circumstances. 

It requires a regular pin if you have buometrics set up, doesn’t shpw notifications

On my Samsung device I had to enable “Power Off Menu” manually. I can now lock my biometrics by holding down the power button.