Systemic fluoride is especially helpful for young children because it can strengthen teeth while they're still forming. That way, they're protected from decay before they ever come in contact with food. It's protection from the inside out. Even if you have all your permanent teeth, systemic fluoride can be helpful because it ends up in your saliva, which then offers an ongoing protective fluoride bath to your teeth.
Topical fluoride is the kind you find in mouthwashes and toothpastes, and in the varnishes, gels and foams you might get at the dentist's office. Topical fluoride protects teeth from the outside in, which also helps them withstand the effects of cavity-forming acids in the mouth.