There is NO purpose in total AD scalings.
Please note: Not a single damage value or scaling in all of League of Legends is changed by this proposal.
TL;DR. % Total AD can be broken up into % Base AD and % Bonus AD. The base AD component can be grouped in with the base damage of the relevant spell. This is a good thing because it makes base damages and scalings on these abilities tweakable independently, all whilst not changing a single value in LoL.
ORIGINAL POST:
What is the point of total AD scalings when they can be converted to a higher base damage + bonus AD %? Yes, you used to be able to change base AD with Sterak's, but now it's a defunct mechanic on SR after the Sterak's rework.
Why bother? Because it makes difficult champions easier and fairer to balance. Let me explain.
If you want to buff a total AD champion's damage scaling, this is currently impossible without tweaking their base damages also, which tanks benefit from, and this can have the exact opposite effect than intended, as high base damages can mean less need to build damage, particularly when tank items are safer.
BUT. This can be converted numerically identically into a better formula, as seen here. (Skip over the next section if you don't like a little algebra, but I assure you it's sound.)
y% Total AD = y% Base AD + y% Bonus AD
This means that an ability that does
x + y% total AD damage
can also be written a
z + y% bonus AD,
where z = x + y% base AD
This means that you can now change AD scaling without giving free base damages, thus properly incentivising building damage without the unintended consequences of also buffing tank builds.
These cases, where damage goes up per level rather than per skill rank, already exist in the game, and are generally represented in tooltips by white text, so this does not add anything new to the game or the code.
An example of where this would be practical and beneficial.
A prominent case where this causes issues is on my beloved Gravedigger's ghouls, as explained here:
Currently, ghouls scale too well without building AD, and too poorly WHEN building AD. By separating the base and bonus components, you can nerf the base damages whilst buffing the bonus scaling, making the balancing nightmare that is Yorick far more handlable, especially since the MMR discrepancy in his winrate is in part given rise by the reliable scalings in low MMRs but lack of gold-based reward in high MMRs.
The ability to separate ghouls' scalings from bases means that skilled players may be rewarded whilst less skilled may be nerfed, allowing a relative shrink in the MMR winrate discrepancy, and making Yorick a less frustrating champ both to play AS and against.
But, this wouldn't be a fair analysis without viewing the cons too! Just what is it that makes this such a seemingly unattainable move by Riot, when the ability to better balance champs would improve the playerbase's experience?
Actually, there's just one con.
Riot would have to do a one patch overhaul of a number of champs - and nothing intensive, just a shift of numbers.
Yep. That's it. All that stands between Riot and a more balanceable game is the decision to actually do it.
Cheers for reading.