Ok, i'm not sure what you know and what you don't so here's fulll explanation.

PORTB.0=1

high level state to PORTB bit 0 (RB0). This is doing the same thing as HIGH PORTB.0

PORTB.0[i]=1

high level state to PORTB bit specify by i variable

So let's say i=4, this will place PORTB bit 4 (RB4) to high level.

i don't see lots of appliance for this function since i always use specific name to access an PORT pin.

hope this put some lights

regards