Hello Areeb,
Manufacturers have the ability to change the engine strength and weakness^ which can provide an advantage if the manager wants to focus on a certain attribute for their upcoming race(s).
The engine cost can also adjusted, between 500k and 1m per race, providing a financial benefit for the manufacturer. If the engine has 10 customers, all paying 1m, the manufacturer will get several million cash for every race they participate.
^ Only once the manufacturer reaches Level 30, they can adjust the weakness, however the customer will still always have the default weakness, as below:
Acceleration - Braking
Handling - Downforce
Fuel Economy - Tyre Economy
NOTE: Cooling and Reliability cannot be set as the weakness.
Further information relating to Engine Manufacturing can be found here:
Engine Manufacturing Guide.
I hope this helps; if you have any further questions, feel free to ask.