I am not sure it is with an eye towards conservation - that may have been one of the original aims, but it does not seem to be what drives it at the moment. I think part of the original idea was to expose people to a greater diversity of succulents and related plants, so they generally (but certainly not always) have a significant variety of uncommon plants.
The last few years it sounds like it was a combination what they had available through their own nursery program and the Huntington's collection enhanced by specific plants from known other sources - like the Mangaves and some of the Haworthias this year. I think a big part is to enhance the funding for their nursery operation.
The two Mangaves for example can be had at PDN, but through the Huntington are cheaper - one is on sale at PDN so only about $2.50 cheaper, but the other one is $7.50 cheaper at the Huntington and shipping from PDN is about 3x as expensive as shipping from the Huntington.
