When purity goes down you have less active ingredient.
The thing is, a 98% purity when the vendor sends a sample out for testing could be 98% by the time you get it, or 87%. There's just no way to know without testing it again.
And as a practical matter, doesn't change anything because you're not getting a refund.
The starting purity just gives you a rough sense of quality, which you should assume goes down the older those test results are.
I try to get batches with recent test results, then, the second it lands, do everything we know preserves the protein from further degradation. Before and especially after reconstitution, when things tend to go down hill fast.