Bandwidth = QPS response size.
Read bandwidth: If you serve QPS and average response is KB: KB = MB/s = Mbps
Write bandwidth: Usually lower. If write QPS with KB average: KB = MB/s = Mbps
Media changes everything: Video streaming at Mbps per user with concurrent viewers = Gbps. This is why CDNs exist.
Knowing bandwidth helps you understand infrastructure costs and whether you need CDN, edge caching, or compression.