Asia is a sunny place. Even in wet season it can be a sunny place. Even if you’re not planning on any time on the beach, chances are you’ll be spending some time under the sun. The sun over Asia is hot, and if you spend days standing around in it, you will get burnt. Carrying a backpack on very sunburnt shoulders can be very painful. The moral of the story is always pack sunscreen and a hat, and try to use them as much as possible.
If you’re trekking, you’ll also be perspiring, so don’t forget to reapply it through the day. Likewise isf you’re planning on cooking yourself on the beach be sure to reapply sunscreen through the day.
Another fine tool for deflecting the sun is a hat. Even if you’ve never worn a hat in your life, be sure to pick one up once you arrive in Asia — they’re cheap and available all over the place.