Oh, OK, I get it now. This gives you nice tight smooth lines, where as just dividing (no modular) puts some distance between the lines.

Not really. Your gaps you're seeing, are the negative parts of the curve.

What's happening is the sine function will repeat forever if you just feed it say the x from get position, you will get the bump and dip of the curve (-1 to 1) and your pattern will repeat approximately every 6.28 (2 x Pi) meters. You can multiply or divide the get position to expand the wavelength, but it's not a very intuitive way.

Using the modulo is more precise and you can isolate any part of the sine wave, so 2 x Pi gives the full phase of the curve and just x Pi gives you half. 0.5 x Pi a quarter of the curve, and so on.