In my opinion it is necessary first read, then mask and rewrite. Or I'm wrong?
No, it's not necessary as the power-on default is defined and if you know how to set all bits you're allowed to do it in one write.
The value 0 simply sets the frequency to 1Hz and enables the square wave output on the SQW pin, leaving the rest of the bits on the default.
Also in no examples is checked the return code of Wire.EndTransmission. Why?
Because many Arduino programmers don't know about the possibility or don't care about error handling.