Sucker Punch is for Dragapult, basically. Pyro Ball plus Sucker Punch will KO Dragapult, which means you can't switch Dragapult into Cinderace and expect to beat it.
The matchup sounds a bit trivial, but it's fairly important to Cinderace's role. If you're using Cinderace as a form of speed control, then you'd be disappointed if it matched badly against the tier's most prominent fast Pokemon. I suppose it's useful in a pinch for things like Zeraora and Scarf Dracovish, as well.
Most coverage attacks are not really concerned about covering weaknesses to begin with. More typically, you simply use whatever allows the Pokemon to hit the largest range of targets (especially if those targets wall your STAB, like Dragapult does in this case). There's often overlap between your defensive weaknesses and gaps in your offensive coverage, though.
