The advantages of a suction excavator follow along the same lines as a hydrovac with a couple of key differences. By removing the need to use water during your excavation you not only remove the hazards that come along with it, but you also end up with an excavation site that is dry and ready to be worked in if necessary.
Furthermore, because there is no slurry being generated, the materials being excavated may not need to be disposed of at an offsite facility, but can be stored on site for later use, or they can be used to backfill the excavation directly.