So updates could come from the wsus server or a workstation in the domain that has already gotten the update from your wsus, so no increase in internet traffic just local network traffic. I think if you read higher up in the thread they are discussing sourcing from other local network sources not the internet. But hey man, it happens with some organizations. I know it sounds unrealistic to have a high workstation count but a smaller pipe (internet). So when all these 100 workstations are powered on, well hello traffic.
Sometimes when 100 computers are setup for example, a customized image may be from let's say May 2018 and may not have all the updates. This maximizes our internet pipe if for say, 2500+ workstations all dial out to ms-update. So our organization would let WSUS download all the updates and then be a source for retrieval of updates as well as configuration. Why? To alleviate bandwidth pressure due to high traffic and consistent barraging out to MS and causing high traffic. We want our workstations to download from WSUS server only. Every use case is different and not one scenario works for all.
