Some people claim that ranking ubers is pointless or useless since most ubers carry their own specific function, and comparing them to each other should not be a concern for the average player since you will use different ubers for their respective, different scenarios.
This is a perfect example of a half-truth fallacy. It is definitely true that most ubers serve their own unique function, however this in itself is NOT a valid reason to disregard rankings as a whole and call them useless.
The biggest aspect of the game that rankings can impact for an average player are their rolling habits. Rankings can guide/ direct players into aiming for specific ubers and sets which will better their collection. This is why the ability to analyze units from stats is an arguably important skill. Knowing which units are meta and why they are meta can allow you to roll for the best possible unit for yourself (of course, accounting for YOUR personal methodology/ your needs in what kinds of niches or roles your uber collection is lacking in).
This is extremely relevant if you are a seed tracker. To disgustingly simplify it, seed tracking basically allows you to choose what cats you can get from the gacha. Knowing which ubers are better than others is absolutely crucial to maneuvering yourself in the best possible track.
Knowing which units are better than others can also allow you to build better lineups. If you own 2 units that share a similar role, then you can either use both in the same lineup or choose between the 2. The “just use both lol” argument can be applied in quite a few situations (evident with strats such as dual Ururuns) but is also flawed in that it takes an overwhelmingly neutral stance not to mention it can detriment the player into wasting unnecessary slots or creating poor synergy which could’ve been optimized by replacing one of those units for another unit with a different role for better synergy.
One of the best examples of this is Dark Phono vs Miko Mitama. Both have a somewhat similar role in generalist support and can be used together, however they are never used together when you study perfect lineup archetypes, in fact, lineups such as 7’s perfect lineup v4.2 phased Mitama completely out in favor of Dark Phono. This is simply due to the analysis of Dark Phono exceeding Mitama’s capability as a versatile generalist support among other advantages, and as such the ingenuity of rankings (even among a 2 unit comparison) has allowed for players to build better lineups and play the game more optimally.
Ultimately, rankings provide a means of discussion and offer conclusions to methods in how to play the game more optimally whether that’s rolling habits or lineup building.