1. They're incredibly cheap to store. In Prometheus, is may cost you as little as 1 byte per sample (ignoring series overheads). Because they're cheap, you can keep them for much longer and use them for long-term analysis of traffic, resource use, performance, etc. Most tracing vendors seem to cap storage at 1-3 months while metric vendors can offer multi-year storage.
2. They're far more accurate that metrics derived from wide events in higher-throughput scenarios. While wide events are incredibly flexible, their higher storage cost means there's an upper limit on the sample rate. The sampled nature of wide events means that deriving accurate counts is far more difficult- metrics really shine in this role (unless you're operating over datasets with very high cardinality). The problem only gets worse when you combine tail sampling into the mix and add bias towards errors/ slow requests in your data.