It’s part of the monetary policy of the US Fed (RBI of USA). These rates determine how much one can earn by keeping a fixed deposit with the bank - since Fed reduced this rate, anyone putting money in fixed deposit will earn 0.50% less.
This will force people to move their money to places where the returns are high - like stock market, other businesses, investing in bonds, etc.
By doing so, they will increase the money circulation in the economy - which will boost spending.
When spending increases then GDP increases (nation grows). Also, when spending increases then cost of goods increases (inflation) therefore Fed can only do it to an extent. It’s a balancing act between inflation and growth.
There are other technical nuances involved here but this is the stripped down version.