I don't get it why it is important... [...]
Fortunately, other people who are more experienced and more knowledgeable on this particular topic
do. Regardless, I'd implore you to either do some reading into it, or, if you don't
wish to do so, then
accept what's said by the people who understand better.
Here's a rough, specifics-light description of
one of the issues.
If a feature affects a single damage roll, and you arbitrarily and unevenly decide to make it apply to many rolls in some cases and only one roll in others, you're throwing a lot of things out of whack immediately....
You want an AoE damage spell to make individual rolls per target at casting - okay, but you've just weakened several class features substantially by doing so. You want to apply that feature to all rolls in that specific case; how do you convey that you are doing that simply but clearly to the player, so they know and can act accordingly? Do you change the feature in question to say the damage of 'one spell or ability', instead of one damage roll? Do you clarify what counts as an ability, class feature, feat, perk or other feature? What ones of those are in and what ones are out for this? How do you convey that simply and clearly to the player?
Suppose you can, and do... Okay, but now you're throwing balance under the bus in an entirely different way because you're making that feature apply multiple times to individual targets by way of it applying to each individual roll of other spells and abilities, such as scorching ray, which can all target one creature; maybe that's fine, but it's also incidentally making the rough power grade and equity of some class features hugely out of line and more overbearing than others; what do you do about that? Do you change the other features that are now underwhelming by comparison to bring them up? How do you do that? Do you manage it without throwing further elements out of line?
If the solution was as simple and trite as some folks make out, such that it could be suggested by anyone with a base-line degree of human intelligence, it's safe to presume that the folks responsible for making the system, and who have a very real vested interest in making a system that is fun to play and will be successful, and who are employed to do so over others because they have at least a certain amount of qualification or aptitude for doing so above the average person, would have hit upon it already? Of course they would have; it's just not that simple.
Rather than being so dismissive as to suppose that a banal one-line answer is magically a good solution but is also something that has not been thought of by the very people designing the system as their day job.... you could implement the spell as written, and realise that the
majority of spells and class features are the way they are for a reason, and that reason is generally not, in fact, arbitrary, but ties in with the balance of the system which has spent the past many, many years being refined and tweaked.
It's not perfect, but it's a damn sight
better than a group of game developers' impromptu hacks that lack in any kind of foresight or understanding of a system that they very genuinely do not really seem to understand very well at all.
Yes, different creatures may do better getting out of the way of the fire than others, or take better advantage of the options they have in the moment - That's what the saving throw is for representing; whether they take less damage than their compatriot because they managed to avoid the brunt of the attack more effectively, for any and whatever reason...
Sorry, I've had a rough week. If I come across as harsh or angry, I don't mean to...