We have always answered this question with a quick yet effective analogy. Imagine a toaster that is set on a low setting for twenty minutes. Although on low, the bread is getting burnt. The same thing happens to your skin on a daily basis. UV damage from the sun, your computer and the environment more generally accumulates over time. By wearing sunscreen every day, you are ensuring that you are protected, and that the people who care about you most know you are doing your best.
Men, specifically, are more likely to develop skin cancer than any other cancer and wearing SPF daily reduces your chances of skin cancer by nearly 40%. Sun hat, sun shirt, sun glasses and shade (say that 4x fast) are recommended for a comprehensive sun care program.