Do you really think the SNB couldnt print a quadrillion franks tomorrow and drive it into the ground if they wanted to?
Doing this is always a risky policy. SNB has told many time that they will use unlimited means to keep the value of the Swiss francs however in 2015 they had to give up on the promise to keep a 1/1.2 EUR/CHF exchange rate.
What history told us is that even hedge fund (like Soros in 1992) can bring a central bank to its knees.
Why not spend some on infrastructure/whatever?
Maybe they use the reserve as some sort of geo-political insurance against possible shocks (whatever those might be)
They then exchanged them for the USD they now hold.
As for infrastructure/whatever, SNB then went on a massive shopping spree in equities, ~100 billion or so.
They must be the absolutely best managed central bank in the world.