NinjaZ'Oh god': There's a buried Steam help page that shows how much money you've ever spent on the platform, and you may not want to know
vxxHome - > Account - > Purchase History
I don’t know why you would use a third Party Tool that estimates your purchase 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 and donations and just applies a price to it.