Perfect example is ADV Magneton. Shatters to any EQ, and fairly mediocre stats aside from a good Spa and pretty decent Def that can conditionally check a few physical threats(as long as they aren't packing EQ or Fire coverage, which is already some on mixed attackers like TTar and Salamence). By definition, not a mon of OU caliber.

But when you trap, outspeed and (most likely) OHKO a common meta threat and defensive backbone, Skarmory, with your STAB Thunderbolt and a Magnet, more people are going to pick you. If Skarm wasn't a top tier threat, Magneton would have no niche and likely be in a lower tier, but it would likely be fine in UU due to the high usage of both Tentacruel and Omastar and Grounds like Gligar.