I'll start with what DNA Circles used to do.
|A screenshot from Ancestry's blog of how DNA Circles|
showed everyone in the group shared DNA with each other
So, DNA Circles was primarily looking at the shared group DNA and only using trees to identify the source of all that shared DNA (which is actually much how Genetic Communities work too, but I digress).
ThruLines works sort of in the completely opposite way.
|ThruLines is only showing these matches descend from the|
same ancestor based on trees - it does not tell you whether
they share DNA with each other or not
What does this mean? It means that beyond the fact that ThruLines is only looking for tree connections with your DNA matches, DNA really isn't a part of ThruLines. The groupings are not based on DNA like DNA Circles were because ThruLines doesn't even know, much less show you if the people in the group share DNA with the others in the group or not. Knowing which of your matches also match each other is important for establishing a connection to an ancestor because remember, family trees are fallible and you can't rely on them alone, especially when there's also no known paper trail to confirm it. You need those DNA groups/circles to tie those alleged descendants together and confirm what the trees say, but ThruLines doesn't do this.
|Another screenshot from Ancestry blog showing the now|
retired New Ancestor Discoveries.
Hopefully that helps clarify the main differences between these two tools. They are/were both useful in their own ways, but they are different and understanding their differences is important so you're not making assumptions about ThruLines and letting it lead you astray.