The soil in raised vegetable garden beds typically warms earlier in spring than the surrounding earth. It also tends to dry faster, so you can get cool-season crops planted sooner, allowing for greater choice on what you can plant.
Enclosures will protect frost sensitive plants from those late spring frosts that can damage or even kill new tender plants.
Planting early allows roots to get established and grow deeper to so plants require less water when the weather gets hot.