Why Some Fibres Need Water to Function
Gel-forming soluble fibres, psyllium being the clearest example, work partly by absorbing water, which is what produces the gel. A psyllium-based product used without enough accompanying liquid isn't working the way it's designed to. This is specific to gel-forming fibres; fibres that dissolve clear without gelling, like resistant dextrin or inulin, don't have the same direct mechanical relationship with added water.
General Fluid Intake and a Higher-Fibre Diet
Beyond the specific gel-forming mechanism, adequate fluid intake is a reasonable general habit to pair with increasing fibre intake, since fibre affects stool bulk and consistency, and fluid plays a role in both. This is general dietary advice rather than a claim that a specific additional amount of water is required for every fibre type or every person.
The Practical Version
Following a supplement's own usage instructions for liquid volume (most fibre supplements specify a minimum amount of liquid per serve) and generally staying adequately hydrated covers the practical side of this for most people. It's not a reason to avoid fibre, or to treat water intake as a separate project; it's a normal part of using a gel-forming fibre as intended.