Any ideas what's this for? I've searched, couldn't find anything.
There have been other items like this. In Gen 4 you can get a box from a Team Rocket member that gives you a TM in gen 5. The only thing we can sorta know for sure is that it is some sort of event item but as to what it does we can only guess. It may be a year or two before we know what Gamefreak intends to do with this or it may just be nothing at all.
While the item does nothing as of December 2016, the description has been changed in Pokemon Sun & Moon:
An ornament depicting a mysterious Pokémon that has been venerated as a guardian deity for an extremely long time in the Alola region.
So we know that it represents a mysterious (perhaps Legendary) Pokemon in the Alola region.
Here's my theory:
We know the Strange Souvenir is from the Hiker's own region, a currently unknown and unnamed region. The Strange Souvenir, as it's description suggests, is a Pokémon that is hailed as a protector of sorts in that region.
The Strange Souvenir looks a lot like one of those Easter Island heads, or moai, doesn't it? Some historical theories suggests moai were the "embodiment of powerful living or former chiefs". Other theories suggests moai are "protectors of Easter Island".
Easter Island's actual name is Isla de Pascua. It is a special territory of Chilé.
Generation VII's real-life location confirmed.