The critical density of the universe is (3c2H02)/8piG which is the average energy density required for flatness. And in fact recent measurements imply that our universe is indeed flat and therefore has this energy density. In addition, the Hubble parameter H0 has been determined in the past 5 years with remarkable accuracy and is 71 km/s per Mpc plusminus some modest uncertainty. So what is the density of our universe? We live here and ought to have some idea what it is. Presumably you know c, and G, and pi, and 71, so it should be a snap for you to guesstimate the rough size. Knowing this density can be handy because it lets one compare other energy densities to it----like the energy lost from the CMB thru expansion----and figure out what various fractions of it amount to, like the part of the energy that is visible matter and the part contributed by radiation and so on. But it's good as well because it gives a feel for how thinly or thickly space is occupied.