I'll make an observation: I write C# for a living. The great thing about that is that it has a truly great debugger. But, as you rapidly discover, it's easier to debug some code than others. For one thing, you want to be able to go back to the start of the function and re-run it. That means that methods that mutate internal state are hard to debug. Also, it's even better if you can follow the chain of reasoning without rerunning the code. This means having a variable for each assignment, rather than overwriting an existing one. Finally, when processing large data lists, it's easier to debug if you have separate variables for logical steps. e.g. Get the employees of Company X (variable) that are managers (variable) and sum their salaries (variable). Trying to debug round a for loop is an exercise in frustration.

What all these things have in common is: it's easier to reason about the values in a program than the program counter, and that destroying information makes it harder too. And that, to a great extent, is why FP is useful. Even really basic FP in C# or Java.