Hi Sony,
The key here is final characteristics.
It is easier to influence what the project will deliver in the beginning, before scope is set and the budget is allocated for the work. The closer you are to project completion, the more costly and difficult it will be to change the planned work. If you think about building your house, you can plan to put the bathtub in the master bathroom anywhere you want during the design phase. Once the pipes and drains have been installed, however, it will be more costly and potentially cause significant delays in subsequent completion dates to move it to the other side of the room. It will be EVEN MORE difficult once the floor tiles are laid.
I hope this helps!
Best regards,
Tracy