Several properties, such as 'transform', 'background-*', and '*-shadow', take complex ordered lists of values. Tweaking one component of such a value (e.g. changing color or rotation on :hover) requires rewriting the whole thing.
It is also desirable to be able to decompose complex functions like
linear-gradient(). Such functions are also lists (tagged n-tuples to be precise). One of practical tasks sounds like “transition of 4th color stop from color 'red' to 'yellow'”.
In general the 'transition' shall accept only atomic values to avoid any ambiguity. E.g. it is not clear what exactly transition from