I don't know why you would use a third Party Tool that estimates your purchases, when it has always been right there in your account, without estimates.
Main reason for me is that I have bought humble bundles, donated to gamejams, and gotten keys off of legit and grey-market sites in the past in conjunction with buying directly from Steam. Those aren't included in the Steam spend category.
The tool doesn’t know how much you paid for it, though, so it's completely ignoring sales, donations and in app purchases, and just applies a price to it.
This isn't a 3rd party tool, it's a separate Steam Support page that lists your total purchases. It basically takes the data from the Purchase History section (assuming that you usually pay directly and not using Steam gift cards) and totals it so that you don't have to do that manually.
It was part of the Valve Orange Box and that was a big deal at the time. There was also a huge deal of whining from people who paid for it when Valve announced they were changing it to a free to play model.