When it is just one person working on a project, that person can hold everything that needs to be done in their own head, and can simply execute. This lets the spend 100% of their time on actively furthering the project.

If this person is working 8 hours a day, then they're spending 8 hours a day actively furthering the project.

Now, if this person hires one more to collaborate with, neither of them can work the full 8 hours because they now need to co-ordinate with each other on what needs to be done by whom, what has been done so far, etc. 

This co-ordination brings down the efficiency of both of them as compared to when they were working individually. 

So, if each of them spends half an hour a day in co-ordinating and communicating with each other, they're spending 7.5 hours each furthering the project. 

Nevertheless, 15 hours a day is better than 8 hours a day. That's a huge gain in output for a little loss of efficiency.

The more people you add to the project, the more that communication overhead becomes, and lower the efficiency that is realised. 

So, beyond a point, it is actually worse off to hire another person to work on the project. 

This drop in efficiency though, is not a constant. 

A skilled person can introduce predictable processes to bring down this loss of efficiency and maximise the output.

Having routines and processes in our own lives plays this very role of increasing the efficiency of how we spend our time and what we get done with it.