A Priority Pass membership can be acquired as either a benefit of a credit/charge card product or purchased directly on the Priority Pass website.
As result of Priority Pass memberships with increasing numbers of credit cards, the number of memberships has greatly expanded.
Initially airport lounge operators responded to the accompanying expanded demand by creating additonal blackout times for members who acquired the membership bundled with a credit product.
During that time, those additional blackout times could have been an edge case where purchasing a membership outright might have made sense versus receiving a membership as a benefit of having a credit product.
Now, however, expanding blackout periods exist but to the best of my knowledge those blackouts are without regard to how one acquired his or her membership.
Given that there is now little, if any distinction, between whether one's Priority Pass membership were purchased directly or bundled with a credit product, there is little case for directly purchasing a membership directly.
Below are edge cases where you could plausibly justify purchasing a Priority Pass membership directly instead of bundled with a credit card, but most people would be better off waiting for either or both of the conditions to be resolved.