The waterfall methodology is very powerful. It simply states that first one should think about what is being built, then establish the plan for how it should be built, and then build it. It allows for a software engineering methodology which is more in alignment with hardware engineering methods and practices.
It forces a discipline process to avoid the pressures of writing code long before it is known what is to be built.
Many times an implementation team has built some code to find out that it was not needed or will be of little use to the end product. This early code becomes a legacy and too costly to abandon. The waterfall methodology forces analysis and planing before actions are taken. This is good advice in many situations.
The process forces the analysis team to precisely define their requirements. It is much easier to build something if it is known what that something is.
Significant numbers of software systems have been built using this methodology. Many corporations owe their success to this methodology.