Worms get inside chickens. Even if you treat them, they get it. And if you treat them too much, the worms can get resistant. They're pretty gross, too. You'll see them in poop, looking like short pieces of angel hair pasta. Even if you don't see worms, it's a good idea to get regular fecal checks.
Liquid or pills: The choice between liquid and pills will probably depend on the size of the flock. Pills are generally best because you can control the dosage and make sure it's right for each specific bird. With liquid, you put the medicine in their water and hope the affected birds drink enough. The pills and liquid worm medicine are available at feed shops or online. The pills we get are one per four pounds. Since we weigh our birds every week, it's easy to know the exact dose. We cut the pills to be the amount we need.