Formatting Memory Cards?

whenever you buy a memory card for a console second hand do you:

delete all the saves on it to start fresh, or keep the saves?

kinda curious what everyone else does because I recently bought a black PS1 memory card to go with my PS2. However, it already has a bunch of Final Fantasy saves on it where the previous owner had spend 99 hours on it apparently. I think for that reason alone, I feel a little bad about deleting them xd.