Most DSLR cameras have pre-set shutter speeds in 1/3-stop steps, 1/10, 1/13, 1/15, 1/20 etc...
For time-lapse when the light is constant, using a fixed shutter speed is fine. But if the light is changing, the exposure must change.
In a time-lapse with changing light those big 1/3-stop steps show up as distracting changes in brightness.
It’s much better to use the shutter in bulb mode because you can change the shutter speed in very tiny one millisecond steps - this is called bulb ramping.
Thank you to Adam Shomsky for the slow motion shutter video.