In theory, so that we can spread them around a bit, and actually get to use them once the bombs start flying, rather than the whole stockpile being destroyed in a first strike.
Assuming we're not already glowing silhouettes by then anyway.
Isn’t the submarine fleet the main platform for the nuclear deterrent? There’s 4 of them, and I doubt they’ve remodelled the interiors to hold an extra 20 nukes a piece.
In what situation is it better to have 260 nukes than 180?
180 x 100 kiloton warheads can surely reduce half the world to a wasteland already