you put it on problem areas (fender wells, frame, engine mounts, etc.) because they get hit by debris and scratched up and then in the winter the road salt causes them to rust. it takes decades for severe damage to occur, but i prefer no damage. also the undercoating is rubberized to provide durability and flexibility. i dont do this every year, but i noticed some rust down there and also i just trimmed my fenders so all i have down there is some primer to prevent rust (which will wear off after a few months). AND as an added bonus, it provides a little extra sound proofing around your wheel wells if you completely coat them.
also a note, it should go on rough, but not goopy.
also a note, it should go on rough, but not goopy.