For the last few years, the interest rate of Japanese government bond is negative. That is, you give 100 to them, and they pay you back 99 in 5 years.
The top concern other than earning interest is safety. If you put your money under the bed, it might be stolen or burnt down. Besides, large institutions simply can't have billions in bank notes stored in their office. If you put your money in a commercial bank to earn small interest, the bank might fall and you never get your money back.
The treasury bill or other government bonds are backed by the government, who has the authority to print money, so they won't go broke and you always get your money back. (The exception is euro area where governments can't freely print money. Another exception is Russia in the late 90s when they decided that defaulting on their own gov bond was better to the economy than printing quadrillion rubles. Even the US treasury carries the risk of technical default once in a while.)