Here is an example:
myname equ 7
pre data 55h

org 0000h
mov myname,#0ffh ;copy 0ffh in iram (addr 07h.....r7 bank 0)
mov pre,#0ffh (same but iram addr.55h)

pre data 56h ;we have changed the value of pre
mov pre,#0ffh (iram addr. is now 56h)

myname equ 8 ;this is not possible (myname is already set to 7)

equ directive for a variable myname can be used only once in the program, but pre can be changed as many times as you like