I think we have gone over this a few times.
I have the default screen on my Joule. The useful metrics are:
Time, as mentuoned by others to know how much longer, and how far in we are. Also to glance at a few times to get a feel for lap times so I know the time left when lap cards come up.
Speed so I know how fast a break will have to go and whether or not I have a chance of surviving in one.
Power in case I get in a break so I don't go too easy if I'm solo or too hard if I'm with others.