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Isn't that to counter balance the rest of the body ?

One experiment attached an artificial tail onto a chicken and the chicken shifted its stance and was able to walk similarly to how we expect dinosaurs walked.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3915051/

Kangaroos?

Kangaroos are the model of the 'old' T.Rex - they're mostly a tripod

I believe that the idea is that the tails of T.rex and other dinosaurs were held up by non-stretching tendons. It would not require active effort. By counterbalancing the head, the high tail would reduce the energy needed to hold the head up.