Why Does the Top Note of a Rolled Chord Need a Clear Arrival?

A rolled chord is often shown by a wavy vertical line beside a chord and usually indicates that its notes are sounded rapidly from low to high. The problem is not merely whether the sweep is even, but which note carries the pulse and the melody. Yamaha’s example explains that the notes are added one by one and sustained, and even shows how briefly dwelling on the top note can make the roll more melodic. Analysis of historical recordings also finds different timing choices for the same written gesture, so the symbol does not prescribe one fixed micro-timing.

https://hub.yamaha.com/keyboards/k-how-to/poprock-voicings-part-3/
https://www.mtosmt.org/issues/mto.05.11.3/mto.05.11.3.leong_korevaar.php

Suppose a left-hand bass note and a four-note right-hand chord are written on the first beat. If the fingers merely race upwards, the top note may arrive so late that it sounds detached from the beat, or it may be struck abruptly in an attempt to catch up. First play only the bass and top note, deciding their timing and melodic relationship. Then restore the inner notes so the roll leads towards the top, rather than leaving it as a by-product at the end of the movement. The top note need not always fall exactly on the beat; its arrival needs a musical reason and should be broadly repeatable.