As noted in the investment order pin, an HSA is usually the 2nd place to put investment money. If the account is used for medical expenses, the money is never taxed. That is better than either tIRA or Roth, since those are taxed once, at the end or up front.
One possible reason to not use it is your income is low, and you need the social security credits. The HSA is also not subject to social security taxes, but this income is not counted for purposes of collecting a benefit in retirement either. The SS credits are 32% for income up to around 65k, and 15% for income 65k-130k. Some people might want that 32% credit. I have not done the analysis to see which is better.
Other reasons to not fund it right away are that you want to save for a house down payment in a Roth instead, pay debts, or your 401k is all you can afford.