Why do conifers have leaves like needle?

Deciduous trees are trees that lose their leaves. Trees that don't lose their leaves are "Evergreens".

Evergreen trees have needles, instead of flat leaves, to survive winter hardship.

Needles cut evaporation so trees can save water - dear in the winter.

Not all trees that bear needles are evergreen, for example the larch drops it's needles in the Autumn.