Width enables the practitioner to measure Yin (substance or mediumship). It is the Yin axis of the pulse.
The width of the pulse is provided by mediumship. In Daoist medicine it is considered hubris to determine that a pulse is too small or too big. Therefore the assessment of adequacy is made based on what you feel the pulse wants to do. Does it feel as though it wants to become bigger? Then it’s thin. A Wide (Big) pulse is uncommon today because as a culture we don’t cultivate our capacity to produce Yin.