over a simple password screen lock: nobody can take the internal memory of my phone out of the phone itself...
They don't need to. There's literally YouTube videos about how to do this. Any average person with dedication could do this. Any thief that's made it their life work will thank you for not encrypting your phone, because it'll reduce their effort to mere "minutes" instead of "hours".
We're not living in the Stone Age. This is the age of prolific technology everywhere. If you're not using full drive encryption on your phone, laptop, etc, then your data is very easily accessible. Yes, there is a minor performance penalty for encrypting your phone, and some minor inconvenience, but using encryption is a major inconvenience to those that might gain access to your phone for a short period of time, like an hour or even overnight.
While some critical information is encrypted (typically passwords, PINs, and the like), most of your data is opaque, and can be used to commit all sorts of Bad Things™ with that data. By using full device encryption and a secure password, you avoid being a casual victim.
No security is absolutely perfect, but using a PIN instead of encryption is only protecting the "front door" of your system. Many phones can be accessed through back doors, such as SSH servers, safe mode kernels, and more. Many devices simply do not get security "right." Full device encryption helps bridge that gap in security.
There are still some risky, but theoretically successful attacks you can do against encryption, but those are still outside the reach of casual thieves. Phones without encryption, however, can be broken in by anyone that actually bothers to get a hold of your phone with the intent to get your data. Phones with encryption are usually only good for one thing: reselling on the black market. In many cases, your data is far more valuable than the device itself.