When it comes to saving for medical expenses, you can save some in an HSA or you could save it in some other investment like a bank account. The optimal choice involves complex considerations. Some are:
Contributions to an HSA requires you to have a high-deductible health plan. The HDHP will have, generally, lower premiums than a plan with low deductible, but it may not be always true. Depending on your employer's benefits and your utilization, the HDHP may not be the best option.
For example, my former employer ran a medical system. The plan that restricted you to their facilities had no deductible, and had the lowest premiums.
Contributions and withdrawals for qualified medical expenses from an HSA are tax-free. This is the main benefit over the savings account. However, note that medical expenses over 10% AGI (2019, was 7.5% 2017-2018) are deductible anyway. Thus, if your AGI is low, the advantages of a HSA go down.
HSAs are held in insured bank accounts, essentially checking accounts, so any interest paid is miniscule. Worse, custodians can charge monthly fees, check fees, statement fees, etc. Other investments may have better returns, offsetting tax benefits.
Non-qualified withdrawals count as ordinary income plus a 20% penalty (with limited exceptions), and, unlike 401(k)s, you cannot borrow against it. If you no longer have an HDHP, the money still remains in the HSA. This will pose a problem if you find you need the money for something else.
In sum, saving money in an HSA may or may not be the best thing for you. You really have to consider a large number of factors, and unfortunately, you have to try to predict your future situation.