Can someone clarify why we need to calculate point 3 below. What is the use and how it relates to Design consideration.

Below are typically calculated as part of Back of the envelope estimation

1. Storage capacity : This is used to calcuate storage required, say for next 1-5 yrs.
2. Read Write ratio : This is useful to calculate if we need to have multiple read replicas, if it's more read intesive.
3. Bandwidth : I'm not able to undestand the purpose of this, I'm copy pasting a sample calculation from a popular site.
   Say, the app is hosted in multiple regions, based on the request origin, nearest load balancer will hit, and then the request will be redirected to the Web/API servers. I'm guessing the bandwidth in this case relates to the LoadBalancer and not the actual servers. Even at home we have close to 250Mbps and upload 10mpbs, does this calculation even make sense for a compay hosting an app, isn't it trivial?

Bandwidth Estimation: If our service is getting 2TB of data every day, this will give us 25MB of incoming data for each second.

2 TB / 86400 sec ~= 25 MB/s
Since each incoming message needs to go out to another user, we will need the same amount of bandwidth 25MB/s for both upload and download.