I want to expand on using Ditto to scout movesets, because it's really cool. For example, let's take something like Deoxys-S, which can be at times a nightmare to gauge a moveset from based on Team Preview. By leading with Ditto while your opponent leads with Deoxys-S, you nearly guarantee yourself an advantage; if it's a set like Magic Coat / Taunt / Stealth Rock / Spikes, then you pretty much immediately block your opponent from setting any entry hazards or Taunting for fear of Magic Coat. And with Choice Scarf you're faster than their Deoxys-S, so if the moveset is something like Superpower / Taunt / Stealth Rock / Spikes, then you can Taunt to guarantee their hazards don't go down or lay Spikes of your own. It's actually quite a useful lead, getting information about the opponent right off the bat, and if you know more about the opponent's team than they do yours, you have an inherent advantage.
Another example is checking Pokemon for sets that could be troublesome to you or hinder a sweep; if you're trying to sweep with LO Volcarona, knowing that your opponent is a Double Dancer Terrakion, and not Choice Scarf, allows you to go on your offensive plan more quickly with less need for sacrifice in order to scout your opponent, while if you do see a Choice set and can reasonably assume that it's Choice Scarf, then you now know that you have to manipulate your opponent into losing Terrakion in order to win with Volcarona, and can focus on that to get the Volcarona sweep going as soon as possible.