What are the benefits of bed rest?

Unclear: Randomized trials to date have not shown bedrest to benefit women in terms of preterm delivery. Activity likely raises blood pressure in women with hypertension of pregnancy, though this is likely more of a downstream effect than an impact on the actual pathologic process.

Bed rest: I am going to assume you are referring to bed rest during pregnancy. The answer is that there is very little, if any, benefit to bed rest in improving pregnancy outcome.