It is always tempting to think that other people's jobs are easy and that they "earn" money old rope.
Some people are reluctant to pay a builder to do a job when they can engage separate trades themselves and cut out the "middle man". This is okay if the project is straight forward, if you know how to develop a building, and if time is not critical, but you can compare the main contractor with the conductor of an orchestra, able to co-ordinate the different trades, materials and machinery to arrive and depart at his request with minimal delays between each to finish within an agreed programme.
Co-ordination between trades is vital to achieve a smooth-running and well-built and finished building. How one trade leaves it's work to receive another trade can make a big difference to the running of the project.