Question

Why are leaves arranged in different ways on a stem or a
branch?

Answer 1

Ferns have fronds.  

Conifer leaves are typically needle- or awl-shaped or scale-like.

Angiosperm (flowering plant) leaves: the standard form includes stipules, a petiole, and a lamina.

Lycophytes have microphyll leaves.

Sheath leaves (type found in most grasses and many other monocots)