Bead on buck regulator circuit
Ferrite beads are typically used to filter out RF noise, in this case most likely noise coming from the SMPS going into the power source.

For lower frequencies, they have L00, but RF has the unfortunate property of considering large coils as arrays of capacitors (think of each winding turn as one side of a plate capacitor) and jump them. So, you often needs an intentionally smaller one to combat that.