The three levels

Low effort favors speed above all: reach for it on a task you already understand well and mostly want done quickly, a small, well-defined fix, a mechanical rename, a question you could answer yourself but would rather not type out by hand.

Medium is a reasonable default for most everyday work: enough deliberation to avoid an obviously wrong first attempt, without noticeably slowing anything down in the process.

High effort is worth reaching for specifically when a task is genuinely ambiguous, involves several interacting pieces, or is the kind of thing where a wrong first guess would be expensive to unwind later, a broad refactor, a bug with more than one plausible cause, a change to something security-sensitive.

None of the three is a strictly better or worse choice in the abstract; each is calibrated for a different kind of request, the same way you would not apply the same amount of caution to renaming a variable as to changing how authentication works.