Tips from 1-800-PetMeds:

Benefits of Using Tick and Flea Preventives
1. Reduce skin allergies, hot spots and infections. Many pets are allergic to flea saliva, and this can trigger allergies, itching which leads to scratching. From persistent scratching, this will lead to hot spots and other skin infections. To cure hot spots, use a soothing spray.
2. Prevent tapeworms. 
Using a flea preventative can help reduce your pet's risk of tapeworms, which are often carried and transmitted by fleas.   
3.Prevent tick-borne diseases.By using a preventative that kills ticks, you can help to reduce your pet's risk of Lyme disease, Tick Paralysis, Tularemia, Canine Ehrlichiosis, Basesiosis, and Rocky Mountain Spotted Fever.   
"We live in an area where Lyme disease is common and our Beagle drags her ears and picks up ticks while out walking. She had Lyme Disease once while using another product, so we switched to K9 Advantix and had no re-occurrence for the last 5 years." - Beverly
4. Prevent a flea infestation at home.Nobody wants to see fleas in their home! It means tedious cleaning, vacuuming, and fogging to rid all fleas from your home. Using a preventative can reduce the likelihood of fleas not only on your pet but also in your house.