SPF, or sun protection factor, describes how much longer protected skin can resist reddening than unprotected skin - if you're wearing SPF 15 then you can theoretically spend 15 times longer in the sun without burning.
It's true that there is a minimal difference in coverage between SPF 15 and, say, SPF 45 if you apply the appropriate amount of each. But studies have shown that most people apply about one-quarter of the sunscreen they should, and get only 20-50 percent of the SPF they think they're getting. For the record, you should apply about a quarter-size amount to your face, neck and chest - use that as a gauge for the rest of your body.
That's why higher SPFs are so valuable: Even if you don't apply enough, you're still better protected than you'd be if you used lower SPF. But you still need to reapply - every hour during direct sun exposure!