Warrant is long term right in which warrant holder can purchase certain number of common stock at pre-determined price within specific duration. It is right option to buy the common stock not the obligation. It is also called sweeter because it makes debt and preferred stock more attractive and marketable. Warrant is attached with bond and preferred stock generally to encourage investors to invest in it.
Debt and preferred stocks are less attractive investment as compare to common stocks. When marketability of preferred stock and bond are low at that time warrant can be attached to encourage investors to invest on it. Price of securities attached warrant is higher than the straight bonds.