I can see why it was done, trestles / supports become expensive and a bit of an engineering project as boat size increases, but is it a great idea to have the keel sitting in fresh water most of the winter ?
A serious bilge pump may be required in the trench for any maintenance work, and things like painting ( more, if that's a lift keel ) may not be possible, it will be too wet in the usual spring weather.
As I say I can see why, but storage above ground level would have been prefrable; also the higher the boat, the more deterrent to casual thieves.
The only real advantage I can see is if it's a particularly windy place, I see the other boats do the same, and it isn't UK...