The basic idea is to have an open system instead of closed. The open system exchanges data with spreadsheets.
Suppose I have a Work Breakdown Structure hierarchy with task durations, stored in Google sheets.
It would be flexible and powerful for a user to specify that tasks can come from a range of cells on the web. This post by Google shows how to Import a Range https://support.google.com/docs/answer/3093340
The same can apply to Resources and for Progress too.