why is it important to precisely measure 1 AU?

Could someone explain why a precise measurement of an AU is important?
I faintly remember reading something about how Shapiro's radar
reflecting expermiments (in the 60s/70s) increased the precision of
this measurement dramatically but forgot why it had an impact
(presumably it was needed to precisely calculate orbits so that we
could sent the various explorers to other planets.).

Thanks,

Ted

[Mod. note: because it determines the baseline for parallax
measurements, an accurate value for the AU is also vital to accurate
measurements of distances outside our solar system -- mjh]