The purpose for which the Central Bank sells securities is to

Open market operations are a key tool a central bank uses to control the amount of money circulating in an economy. The bank buys or sells government securities to member banks and the public, and each direction has an opposite effect on the money supply.

When the central bank sells securities, buyers pay for them with cash or bank deposits, and that money flows out of the banking system into the central bank. This withdraws money from circulation, so the purpose of selling securities is to reduce the amount of cash available in the economy.

The reverse action, buying securities, injects money back into the economy by paying sellers, which increases cash in circulation. A change in interest rates can follow from these actions, but the direct and immediate purpose of the sale itself is the reduction of cash in circulation, not the interest rate as such.

Exam takeaway: link central bank selling securities directly to money leaving the economy, and central bank buying securities to money entering the economy.