Why did Tesla sell bonds?

They need the money for capital improvements or some such. 

Same reason any company sells bonds. They need the money.  They were required to file with the SEC and disclose their intended "use of proceeds". Can you read? Nevermind. Clearly not.

...... planning to raise $779.53 million (last month) in the asset-backed bond market. The company’s bonds will be backed by Tesla vehicle leases, made up of mostly the Tesla Model S and Model X.

The electric automaker is offering eight classes of bonds, a majority of which carry mostly top AAA ratings from Moody’s Investors Service. The bonds will mature for investors in 2.7 years or less.

The asset-backed bond move from Tesla is the first time the Silicon Valley-based electric car manufacturer has made a debt deal in 2020.