Why encrypt the device? Android 5 offers two encryption features:


Encrypt the device (phone)
Encrypt the external micro SD card 




I understand the benefit of 2.: if someone takes my micro SD out and wants to use it on another phone/computer, he won't be able to see my personal files.

But I don't see the the benefit of 1. over a simple password screen lock: nobody can take the internal memory of my phone out of the phone itself. So what is the advantage of 1. over a simple screen lock with password + SIM code lock?

In which precise situation would a thief be defeated by an encrypted device whereas he wouldn't be with a standard screen lock with password?
(I'm not speaking about external micro SD for which I already understand the benefit of encryption)

To be more precise: is there a way for a thief of a phone with password screen lock but no device encryption to access the phone's internal data (let's say he doesn't know the password / cannot crack it).
over a simple password screen lock: nobody can take the internal memory of my phone out of the phone itself. So what is the advantage of 1. over a simple screen lock with password + SIM code lock?


Understand that encryption is used to protect the data from unauthorized access (device misplaced or lost or alike situations). If you are sure that the device would not fall into wrong hands ever, you don't need the encryption. 

As for the quoted text: some devices have their bootloader unlocked; some others don't even have this concept (such as MTK devices) in which case a custom recovery (a bare-bone OS) can be loaded into the Android device which would grant complete unrestricted access to your data  if it was not encrypted.

Alternatively, if a PC has access to your device through ADB (think of Android versions prior to JB) than your data is available to that as well. 

Some might even go as far as taking out the emmc or the type of storage in your SoC and use it somewhere else. What would your screen lock do if Android is not booted into? Nothing!


  In which precise situation would a thief be defeated by an encrypted device whereas he wouldn't be with a standard screen lock with password?


A custom recovery now cannot gives access to your data since it remains encrypted. The thief however can brute force the encrypted partition. Depending upon the power of the machine(s) they've got they may be able to find the key to decrypt the data.