I copied below from the petco website for Capstar

CapstarŪ can be taken with or without food, and is safe to use with most other medications. CapstarŪ is ideal for breeders, boarding kennels, groomers, clinics, shelters or multi-pet households - anywhere exposure to fleas or reinfestation is emminent. An increase in itching may occur after given, due to the effect the medication has on the flea's nervous system.

If I use CapstarŪ, do I still need to use a spot-on like Frontline?

Yes - CapstarŪ is a quick kill solution, whose effects only last 24 hours, whereas a spot-on will give lasting protection for at least a month.
If I have treated my pets with Frontline, why would I need CapstarŪ?

I used this on mine once and had the same reaction.