First of all, you need to wear sunblock in order to prevent all of the effects that will pop out on your skin, such as aging faster, wrinkles, dark spots, and skin cancer. If you are exposed to sunlight in your daily activities and you don't wear any sunblock or sunscreen on your skin, your skin will slowly aging and in the following years you will notice some fine lines on your skin even though maybe you are still in your 20s. You will not realize the fine lines instantly, but the effect will be seen a few years later. The most noticeable effect from not wearing any sunscreen is dark spot. Also, when I was 12years old, I took some extra-curricular activities like pramuka (girl scout?) and swimming lesson. Those activities were take time in 12 PM-2PM at friday and saturday and the sun was hot like crayyyyzehhh. I didn't use any kind of sunscreen on my body nor face, and then when I was 13th, I realized that there were black spots all over my hand. At first, I thought I was sick... but then I didn't feel anything for the black spot. I asked a doctor and he said that all of those were dark spots. He also said that in order to remove the dark spots I need laser treatments which take lots of money. oh I wish I could turn back time... I will wear sunscreen back then. 

here's a video that will actually shows how sunblock/sunscreen is important.

Also, I heard that sun-ray can be harmful for our skin if we don't use sunscreen/sunblock because it will leads to skin cancer. According to Young (2009), sun exposure will eventually cause skin cancer whether it is because of our daily activities or outdoor occupation (while not wearing sunscreen).