What is the purpose of buying the 15- year US Treasury Bonds?

Those who are trying to buy upon the 15 year treasury bonds are always trying to look up on gaining an uniform rate of return in the long run and they are trying to also invest into a security, which is considered to be default risk-free as it is backed by the Federal government of United States of America and it will not be defaulted upon, so the principal repayment will be made under any circumstances and it will also offer a higher degree of diversification to the portfolio and it will also provide with better security at the time of the economic adversity, so it is highly important to diversify the portfolio using these long term bonds.