Still waw and ya may serve two different functions. Still waw after dammah and still ya after kasrah are madd letters. When a still waw or ya follows fathah, it is a letter of lin, or softness. This distinction is particularly important in the rules of pausing.
Madd letters: waw in «يَقُولُ», ya in «قِيلَ».
Soft letters: waw in «خَوْفٌ», ya in «بَيْتٌ».
A soft letter produces a gentle gliding sound; it does not automatically take every length assigned to a true madd letter. Its special prolongation may become relevant when a pause produces a subsequent sukun. Treating all written waws and yas as identical would erase a meaningful distinction in recitation.