io series has lower latency and less variation in latency which is helpful for workloads that need high throughout with low parallelism.

Percona has a nice article with various benchmarks https://www.percona.com/blog/performance-of-various-ebs-storage-types-in-aws/

Anecdotally, we've seen more stable performance and faster database restores on io2 vs gp3 RDS Postgres instances (with the same amount of IOPs).