Heat sink for the L298 stepper driver IC
The total volt drop when saturated can be as high as 4.9 volts when taking 2 amps. This is an internal power dissipation of 9.8 watts and is the reason why nobody with any experience of these devices uses them. Even if you are using half-bridge drives, it's still a very poor choice - MOSFETs would be much better and probably won't need any heatsinking.
If your load is expecting to receive 12 volts, with the 4.9 volt drop is will receive 7.1 volts.
See this question and answer for a more detailed assassination of the L298 (amongst others).