Like many flowers of deciduous woodlands, primroses bloom in spring before the leaves have grown thick on the trees so that they get plenty of sunshine. Many other woodland flowers bloom in spring for the same reason including wood anemones oxlips daffodils lesser celandines bluebells early purple orchids and dog's mercury. It is flowers like these that give the woodland floor a colorful carpet every spring.
img source: http://www.haushalts-und-gartenlexikon.de/primel/