what does 2% failure rate per year even mean when it's presented independent of a number of uses per year
This is a good observation. You can look up what the average number of uses per year is. If I remember right, I've seen some condom efficacy studies include that information.
I feel like I'm missing something basic here that would let me see why it's a useful piece of information on its own.
You're not missing anything basic, you're correctly perceiving ambiguity where ambiguity does exist. Even when information is really important, I've found that it's often been omitted simply because products are marketed to the average person, not to nerdy people like me, and most people don't want to think as much as I do. For this reason, I've found it's very important to be careful not to assume that the world is doing sensible things or giving me all the information. They're not just leaving information out, they're also not being held accountable by a world full of people who think as much as I do. Therefore, they can get away with slapping various nonsense marketing claims and out-of-context data on their boxes without people questioning them.
For this reason, I've found it's very important to be careful not to assume that the world is doing sensible things or giving me all the information.
Yeah, this is good advice in general, and it's definitely what I was doing wrong this time.
P/S/A: There are single sentences which can create life-changing amounts of difference.