por si las moscas *
* for if the flies
Expression used here and there by Everyone
It means that you're doing something not because it's necessary, but "just in case".
Note: The origin of the expression goes back to when there were no refrigerators to store food, so that meat needed to be covered or else it would've spoiled and flies would've come.
"Me he comprado un boli más, por si las moscas."
"I bought an extra pen, for if the flies."
10 out of 14 Spanish, Castilian speakers have heard this