Comment by adrian_b

When backward jumps are allowed you can create loops that are much more tangled and incomprehensible than when you are nesting the loop structures of modern languages.

With backward jumps, you can make multiple loops that are not nested, but you could visualize them as a complex graph that has sequences of instructions in the nodes and which has multiple cycles through which the execution may or may not pass and which intersect each other. Good luck to understand how the code works, because you cannot separate parts of it that can be understood independently, like when using the "structured programming" that is ubiquitous in modern programming languages.

Such indecomposable complex multiple loops were not uncommon before 1970 in languages like FORTRAN or COBOL, and precisely this kind of control structures were the reason why the use of GOTO was criticized and considered harmful.