Under-desk storage, fixed or mobile
Storage that lives under the desk, from single drawer units to three drawer pedestals with a filing drawer. The most personal storage in an office and the piece people notice most when it is wrong.
Mobile or fixed
A mobile pedestal rolls out of the way, which frees the desk for other uses and lets the pedestal double as a spare seat with a cushion top. It is the right default for hot desking and flexible floors.
A fixed pedestal is part of the desk structure and adds rigidity. Better for a permanent position with heavy filing, where a mobile unit would just be in the way.
Measure the clearance first
The single most common error here is buying a pedestal taller than the underside of the desk. Measure from the floor to the underside of the desk top, not to the surface.
If you are buying both together, desks with storage removes the question entirely by supplying them matched.
How many drawers
Three drawers with a deep filing drawer at the bottom is the standard specification and suits most desk jobs. Two drawer units suit hot desks where nobody keeps much. Anything more is usually better handled by shared filing cabinets.