The Waterfall Model had its time and piece of the pie (still has in the form of modified Waterfall models) at the beginning of the software development era when software was complex in nature. Therefore, time spent early reduces costs at later stages e.g., a problem found in early stages is cheaper to fix than later in the process because the software has to be embedded in the hardware - no software updates.

We all know that it's not easy to find a good software engineer back then, or a knowledge center (like we do now) to get help from. If one team member leaves in the middle of an ongoing project for whatever reason, good documentation must be there for the new member to get a complete understanding of what's going on.

The Waterfall Model was never perfect, and Royce never claims it is. Instead, he himself pointed out major flaws such as testing only happened at the end of the process.

One thing I want to add is that Royce didn't use the term Waterfall but he said something like downstream. And he was right because we all know that speed of water is very fast but in this model it's too slow. Perhaps that's why he proposed modified versions of the original Water fall model https://www.researchgate.net/figure/Royces-modifications-of-...