In temperate rainforest, the dominant plant form is often coniferous trees (source). However, coniferous trees are also the dominant plant form in a very different climate and form a different biome (boreal forest). Why are broad leaved evergreen trees not so adapted in temperate rain forests as in tropical rainforests? What is the advantage of needle-shaped leaves in the climate with temperate rainforest?
The largest area of temperate rain forest in the world is on the west coast of North America, which has a summer dry season. In other words, at precisely what would be the best time for plants to grow (based on temperature), there's a shortage of water. (Summer vacation visitors to the rain forests on the Olympic Peninsula are often wondering "where's the rain?")
Conifers have small, waxy, and (generally, with a few exceptions) evergreen leaves. All the conifers in the rain forests of western North America are evergreen; that allows them to grow in the cooler months of the year, when there can occasionally be hard frosts (and when deciduous trees are dormant). The waxy, small leaves help them conserve water in the dry summers. A double advantage.
North of about central BC, there's no longer the summer dry season, but by that point the growing season is getting short enough that conifers have an advantage despite the perhumid climate (Prince Rupert, BC is further north than Edmonton, Alberta).
I'll add that this is not the only non-boreal climate where conifers are abundant: consider for example the piñon-juniper woodlands of the American Southwest or the Piney Woods of the Deep South.