These are best for investors who belong to the higher tax brackets of 20% & 30%. For example, the best interest rate offered by bank FDs currently is close to 10% (approx.), & for an investor who falls in the 30% tax bracket, the post tax yield comes to only 6.91%. In comparison, the tax-free bonds are offering a coupon rate of 8.2% - 8.3% (approx.) for 10 & 15 years respectively.
In fact, you could capitalise on the higher rates being offered by these issuances by exiting the bonds once the interest rates come down. Interest rates are nearing peak levels. So, these bonds are providing higher post-tax income along with a likely appreciation in bond prices when the interest rates move down.