From a mead site:
"Tannin helps the brew work more efficiently and adds to the balance of the finished product. It also helps bind the protein and ride it out of the must, providing a clearer mead. Tannin affects the mead by providing an astringency that gives the mead it's 'punch'. You would add tannin to a mead that did *not* have hops, herbs or fruits. They generally don't need it but it does work nicley in a plain mead. You can get tannins from black tea. "
Sounds like it's just another flavor note to counter the sweetness.
A small amount of gelatin finings will reduce the tannins.
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