You are utterly confused.

"if I can't read data from external memory and put it in the code memory... what is the importance from external memory?"
--- Why on earth do you want to put it in code memory, you have just read it.

"I can't move a block of memory from code memory to external memory..."
--- Yes you can

" I can't move a block from external memory to code memory... "
--- correct

"it seems good only if that external memory is being read by another device than the 8051... "
-- balderdash

"If I could do this I could have all the words that I want to display in the XDATA and could read the whole word at once"
--- that is how it is done, have a display output buffer, move your constant text from code memory to the buffer, add your variable data to the buffer, then display it.

Erik