Consider the following method of building 4 features with 4 engineers. Each engineer works on one feature alone. Working this way, each feature takes 4 months to complete.
We build the same 4 features except this time we finish one feature before we move onto the next. Everyone works on the same feature.
Working this way, each feature takes 1 month to complete.
Whats the Difference?
In Scenario A, the customer thinks you are doing nothing for 4 months and then they get 4 new features all at once.
In Scenario B, the customer gets one new feature at the end of month 1. At the end of month 2, the same thing happens. And so on…
This means they get to use and provide feedback on that feature for the next 3 months. They can help you learn if you built the right thing and give tips on how it can be improved. They also get three months more value from Feature 1 in Scenario B than they do in Scenario B.
The amount of work is the same, the overall length of time is the same, the team is the same. The customer gets value earlier!