If you create a Plan based on an existing Team, and you later want to add a colleague from a different Team to the Plan, you can do so... but you'll be adding them to your actual Team as well. In many cases that wouldn't be appropriate. You might simply want them to be able to see a specific Plan, or to have an occasional task allocated to them. - not to have full access to your Team chat etc.
Does that mean that it's always better to create a new group for a Plan, even if the initial members are all the people in your Team? So you'd individually add all those people to the Plan, rather than basing it on the Team? Would that give you the flexibility to add other people to the Plan at a later date, without giving them access to anything other than the Plan itself?