Cuda elec fan

I'm installing an electric fan in my 340 Cuda. The kit comes with a relay to 12V and an ig wire. I'm thinking of running the ig wire to 12V all the time (fused) so the fan can run with the car shut off. This way the fan can continue to cool without the engine running. Can anyone tell me why not?

You can do it but you have to keep an eye on it or it can run the battery down at the worst moment.