Software Project Planning
Planning focuses on the Business problem, not the technical solution.
Planning allows repeatability. You don’t need a recipe to cook, but… A recipe improves repeatability.
A company can choose many places to invest their money:
Because companies want a higher rate of return than a bank offers, and less risk than a casino, they generally invest in new projects. Planning adds overhead cost, but it also reduces the risk and reduces the cost of defects, therefore it increases the value of the software project.
Cost of Defects
The more time between the point when a defect is introduced and the point when the defect is detected, the greater the cost of correcting that defect - if it can be corrected at all.
Project planning allows the team to devise methods for each of these processes, as applicable. The level of formality will vary by project and industry - but the processes themselves remain the same.
Level of Detail
Level of detail increases as the project progresses. This also means that documents containing specific implementation details had to be created after the coding. Planning documents and SRSs that contain great detail were clearly created after the fact.