> Pelliper moves OU, even though it's species mostly serves as rain setters
> Floatzel and Barraskewda remain in UU, even though they're the best swift best swim users at the moment.
I'm confused :P
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This is a pretty common occurrence for weather-based mons historically: the setters themselves often make it into higher tiers more often than the weather-dependent beneficiaries themselves. It happened with Torkoal multiple times over the course of Gen 8 who rose to OU on numerous occasions while weather-dependent beneficiaries like Charizard and Venusaur were always stuck in lower tiers. Pelipper itself was OU in Gen 8 while Swift Swim sweepers like Barraskewda itself were in lower tiers very frequently (even though Barraskewda eventually wound up in OU again at the end).
I can't really explain it, but I think it boils down to the fact that the weather setters themselves are the core and heart of any weather-based team for the weather they summon while beneficiaries like Chlorophyll/Swift Swim sweepers and whatnot are both less essential to a weather team (ie you can feasibly build a weather team without them) and are completely dependent on the weather in question to be viable in a higher tier most of the time, thus they see less use overall because they don't have usefulness outside of the dedicated weather team, and oftentimes are not guaranteed to take a teamslot on every single weather team like the setters themselves are.
In particular, you highlight
both Barraskewda and Floatzel as both tying as the best Swift Swim sweepers in the tier, and in doing so you just highlighted the very reason they don't see the same level of usage as the premier rain summoner, Pelipper, does. Because both of them tie and are functionally very similar, they are basically competitors for the same role on a given rain team, since their function on said teams, to be a fast sweeper and attacker in Rain thanks to Swift Swim and the Water boost, is more or less very similar, and you are not going to be using both of them on the same rain team. You only have one slot for that role, and it's
either Barraskewda or Floatzel in the vast majority of circumstances, the other four teamslots are for other teammates who serve as synergy/cover/support for the team overall, you need wallbreakers, mons who can cover the weaknesses of Floatzel/Barraskewda, and whatnot. Meanwhile Pelipper is uncontested in its role as a rain setter, and it is
always going to occupy a slot on every single rain team as the heart and core of the team as the Pokemon who summons the rain to begin with: it has no competitors for its role and its role on a rain team is so essential that all rain teams will use it.
That basically is the picture as to why you see a weather summoner like Pelipper rising to a higher tier while direct beneficiaries like Floatzel and Barraskewda don't. Pelipper is completely essential to a rain team and has no competitor for its role, meanwhile Barraskewda and Floatzel have limited use strictly to rain teams that use Pelipper and are direct competitors with each other for the same slot on a given team.