Here's why:

* Swell: This term refers to a series of large, rolling waves that have traveled a long distance from their point of origin. They are usually characterized by their smooth, rounded crests and long, gentle slopes.

* Billow: While technically a synonym for wave, "billow" is often used in a more dramatic sense, emphasizing the sheer size and power of the wave. It can also refer to the turbulent movement of something like smoke or clouds.

So, while both terms can be used to describe a large wave, "swell" is more precise and commonly used in maritime contexts. "Billow" can be more evocative and poetic, emphasizing the grand scale of the wave.