Faceshield therefore protects from the main transmission vector. It's not impossible to get infected otherwise, simply much less likely. Your main enemy are getting coughed/sneezed/talked at in close proximity.
Surface spread is likely also extremely limited (source: Prof. Hendrik Streecke who continues to study COVID-19 in the German epicenter of Heinsberg [2]). Money quote from source:
> "Wir waren in einem Haushalt, wo viele hochinfektiöse Menschen gelebt haben, und trotzdem haben wir kein lebendes Virus von irgendeiner Oberfläche bekommen.“
English: "We have studied a household where highly infectious individuals resided and have still been unable to find any living virus on any surface."
All virus residues found have been dead virus material.
[1, German] https://www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus...
[2, German] https://www.deutschlandfunk.de/covid-19-wie-lange-sich-das-c...
In fact, that effect might be so significant evolutionationary that it is the reason we have a seperate nose and mouth, rather than combining the two.
The biggest problem is actually eating hot food - it seems the mouth can withstand hotter foods than the lungs, which get painful if you breathe in the steam from the half chewed food in your mouth.
This study talks through the advantages of adding them to the repertoire for infectious disease control as part of your PPE: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5015006/
I believe face masks are only 95% effective at best and that's only with a proper N95 which a lot of medical staff appear not to be able to get hold of. Hence they're wearing things like Buffs or cycling masks to make do. In that situation, having another layer that can stop droplets etc. from reaching the mask in the first place makes sense to me.
Plus it stops a whole lot of messy body fluids from reaching the mask and goggles which cuts out a swap/clean cycle (again, difficult if there's no spares!)
[Edit to correct 'googles' to 'goggles', FFHS]
(though N99 and N100 respirators do exist)