Meditation benches are a tradition in Japanese Buddhism. The are called seiza benches, because they help you to sit in the traditional way, kneeling on the floor with your knees in front, which is called seiza (“correct sitting”).
For many Westerners, kneeling on the floor is excruciating, but a seiza bench makes it easy. Using a meditation bench will allow you to sit comfortably for hours, without any pain.
Here’s an instructable on building as seiza bench for yourself. You’ll find full meditation bench plans there.
If you’re new to meditation, or want to learn about other ways to sit in meditation, check out this article.