The reason it is done this way is so that if sometime in the future the design was changed and instead of A2, A4 was used then it is a simple matter to change,
ana2 equ 4 ; analog input AN2
rather than trying to find all occurrences in the code.
I use to use this method to setup SFRs. EG,
Code:
movlw (1<<GIE|1<<PEIE|0<<TMR0IE|0<<INTE|0<<RBIE|0<<TMR0IF|0<<INTF|0<<RBIF)
movwf INTCON; enable Peripheral interrupts
but it confused people and so I stopped using it.
Mike.