2004 Jeep Laredo Sealing an engine

Can you tell me why I would need to have an engine sealed. I just purchased this car from a dealer and they told me they would need to do a seal on the car. I was under the impression they were doing a seal on the outside of the car to prevent rust but now they tell me the car wouldn't be read until Monday becasue it is a big job. I asked what exactly they were doing and they said they were sealing the car engine. I just want to make sure this is OK to do and that it isn't a sign of bigger problems to come.

Thanks,