Joyfirst, I have never had pure globe amaranth tea, but I've had amaranth oolong on many occasions. It is said to have anti-aging properties and be good for the skin. I don't know how much amaranth was in it, but it tasted like regular oolong to me. Just be sure you are only using the globe variety for the tea.
Amaranth has many uses, for sure! The flowers keep their color for an extremely long time. They will outlast anything else in a cut flower bouquet...by a month! They dry readily and will dry out without falling apart/off if left on the plant in the field.
Once dried you can harvest hundreds of the tiny little seeds. They are a super nutritious faux grain that is slightly sweet and protein packed, said to be one of the top ten healthiest foods in the world. (If you leave them too long the seeds will drop everywhere and you'll have hundreds of amaranth plants growing up as weeds next spring.)
The leaves are also edible! (As are the leaves of amaranth's smaller cousin, the celosia flower.) You can eat them raw or cooked. They add a nice splash of color to salads and can help you break out of the spinach/kale rut in a pot of greens.