Without pro-tools you see the estimate of how you are related to your matches.
With pro-tools you see this AND you can see how your match is related to your shared matches.
So I am matched to Bob and Joe. If I click on Bob I can see he is estimated to be my 2nd cousin once removed. His profile also shows he is estimated to be Jo's cousin.
Even if Bob and Joe only have partial tree, knowing what their relationship is likely to be means you can identify where they connect and work back from that point. It then makes it easier to identify your common ancestors because you have two points to work backwards to a common point - as you can triangulate. If you are unsure of part of your tree, you can use these matches to construct a separate tree which you know connects somehow to you.
If you have three or four people who are all related to you and each other, you have a cluster you can work from.
It's useful if you don't know an ancestor OR you know a name but can't distinguish which person with the same name you are trying to identify. It also helps to link in people who don't necessarily have a tree at all.
It basically gives you alternative starting points to find a common ancestor.
For me it was game changing in terms of how useful your matches are.
I am hoping that the new cluster feature will also aid with this when it comes off beta and you can customise.
I love it. But you have to really be interested in family history and getting in-depth with tracing ancestors (or have a particular dead end you want to solve). It's helped me break several brick walls I otherwise might now have worked out. It's not one for beginners - it's immediate to advanced. And you probably need to have the time to invest in making the most of it.