How do you learn more during the process of doing some design up front and refinement?

I have to create an application and do a software development report on it (i.e: do the designs, testing, evaluation etc) and I have to write about how the development changed my initial design of doing something a certain way but development made me realise xyz and so I updated the design to reflect the current method. And my lecturer said to write about "how for future projects to consider the value of ‘some’ design up front plus refinement as you learn more during the process".

Can someone give me some insight into what he means by this / what some benefits may be of doing some design up front and refinement?

I will ask but I'm curious to know a little bit more as a conversation starter before I ask. Thanks