Hi, crgstclr.
There are different facets to your question so hopefully the answers below would help clarify:
- Do you gain anything by using an existing Team Site and adding that to the Team?
[Fred] Yes, it allows your team (members) to access files/assets in a central location via Teams (app).
- And if you do associate an existing Modern Team Site to a Team.... that SharePoint site that was created as part of the team doesn't go away right?
[Fred] Correct, the Team site will be retained and updated accordingly when files are uploaded/modified.
- Are there downsides to having two SharePoint sites associated with the same Team?
[Fred] This question is a bit unclear. I'm not sure if you meant associate two SharePoint sites into Teams but the idea of "Teamify-ing" your site means you're users no longer have to be particular about nuances of navigating within SharePoint.
I've posted a tweet about this topic in:
https://twitter.com/fredyano/status/1065294595788021762
Fred
#FredO365Tips #O365ParadigmShift