Tannins are a slightly anti bacterial, and although this may seem trivial on its own, I believe that the degraded water quality from too few water changes in most aquariums (with gradual buildup of TDS), and extended periods without tannins can contribute to chronic diseases like HLLE. The scarring we see (mostly in adults ) that develop over time, then seem to manifest for no obvious reason in older adult South American species like oscars, and P bass after years of appearing perfectly healthy.
And even if tannins are seasonal in many S American rivers, their contribution may be significant during those periods, to long term well being of these fish evolved to I've in tannin infused waters.