Not a stupid question at all! This method works for several reasons:

• Typical soil contains the majority of nutrients in the topsoil layer, which is roughly 1-10in (2.5-25cm) deep. Because compost is added up to 2ft (60cm) deep, roots will grow much deeper to take advantage of the easily accessible nutrients.
• When the soil is kept loose, plants can grow roots through it much easier; French intensive gardening focuses on loosening the soil very deeply and avoids compacting it so plants don't have to expend as much energy pushing their roots through hard soil.
• As a result of plants growing roots deeper rather than wider, they're able to be planted much more densely and grow more produce than market gardening in the same size plot!