Things that are interchangeable can replace each other easily. Interchangeable objects or people can be substituted, and no one would know the difference.
Some things cannot be switched: if you replace spoons with forks, people are going to have trouble eating soup. Other things are interchangeable: if you switch them, nothing will be lost. Gadgets that are interchangeable can be substituted easily: they're exchangeable or standardized. If two workers are interchangeable, they could do each other's jobs. Two interchangeable words mean almost the exact same thing and could be used in the same way. Interchangeable things are versatile, useful — and, most importantly — "switchable."