I'm just gonna try and ignore the posts above me and state my thoughts on team preview.
Honestly, Team Preview just creates a set of mindgames by itself, and depending on what team you are using, there are reliable leads vs. different teams as opposed to others. Take for example the Rotom-W in the OP - it's good against all weather leads (assuming its Choiced), forcing you to go to your Rotom-W counter, which will vary per team. Let's say were against a sand team using Gastrodon. If you assume it's choiced, Gastrodon will work perfectly. However, there is always the chance that it's actually an Expert Belt variant that could kill you with HP Grass (you still don't know if it's a Choice Specs variant). You have to choose whether to keep your own Gastrodon in or go to something else - let's say Celebi, which tanks HP Grass nicely. From here, you could just make an infinite amount of scenarios of what would happen with Rotom-W in the lead slot. Now, that's just a purely hypothetical scenario, but again, the opponent will often expect you to lead with the Pokemon that's in the lead slot (unless there is an obvious chance to set up weather, or hazards from something else i.e. Ninetales = sun). The decisions of what Pokemon you lead with basically decides what will happen in the rest of the match - admittedly this is the same as regular leads in DPP, but the ability to see what's on the opponent's team adds another level of thinking into it.
Speaking of which, if you use Team Preview you can have good guess of what sets your opponent is using, weather starters and Pokemon with single sets aside. Again, lets use this example, Rotom-W - he's on a rain team, and you are up against the same sand team with Gastrodon and Celebi. The opponent makes the inference that Rotom-W's position on this team is to remove counters to rain sweepers on his team, in particular Gastrodon, so it's running Hidden Power Grass. Likewise, the Starmie on the same rain team with few Pokemon weak to hazards (or another Rapid Spin user) is most likely to be an offensive variant with Expert Belt or Life Orb, carrying Hydro Pump, BoltBeam coverage, and Recover. You could, again, apply this to almost any Pokemon, though some Pokemon are almost always going to be running one set (i.e. Scizor tends to be Choice Band, Breloom will be running a set with Spore). You also can take one's knowledge of the metagame as a factor as well. A particular set someone has been seeing on the ladder more than any other set can, thus, be thought of as the most popular set at that time, and the one to automatically assume - unless there are other factors that alter this (see, im getting back to what i said at the beginning!).
In short, Team Preview is really the actual start of the battle, since you're making all these decisions which will affect the battle. Consider it the actual first turn!