Open the answer key in the back of almost any JLPT textbook and you'll find the same thing: a letter. Maybe, if you're lucky, one short line of reasoning for why that letter is correct. What you almost never get is an explanation of why the other three options are wrong which means every question you miss teaches you exactly one thing (the right answer) and leaves the actual source of your mistake completely unaddressed.
That gap is bigger than it sounds. If you picked the wrong answer, you didn't pick it randomly you picked it because some part of your reasoning made it seem plausible. A discrete "the answer is B" doesn't tell you what was wrong with the reasoning that led you to A. You just move to the next question with the same gap in your understanding, waiting to trip you up again.
What a textbook answer key actually gives you
Here's a realistic grammar question, the kind that shows up constantly around N3–N4 level:
図書館で借りた本は、今日中に___。
A. 返さなければなりません
B. 返してもいいです
C. 返さなくてもいいです
D. 返してはいけません
A typical textbook answer key gives you: "Answer: A." Maybe, if it's a particularly generous textbook, you'll get a single line like "使用 なければなりません to express obligation." That's it. If you picked D, you now know the letter you should have picked but nothing about why your instinct was wrong, which is the part that actually matters if you don't want to repeat the mistake.
What JLPT-X generates instead
Every question JLPT-X generates comes with a full explanation of why the correct answer is correct and why each of the other three is specifically wrong not "wrong" in the abstract, but wrong for its own distinct reason. Here's the same question, with JLPT-X's explanation in both languages, exactly as it would appear in the app:
日本語の解説：
正解はAである。「今日中に」という表現は明確な期限を示しており、図書館の本を借りた場合、返却の義務があることを意味する。したがって「なければなりません」（〜しなければならない、義務）が文脈に最も合う。
Bの「てもいいです」は許可を表すが、この文脈では返却が任意であるかのように弱めてしまい、期限のニュアンスと矛盾する。
Cの「なくてもいいです」は「返却の必要がない」という意味になり、「今日中に」が示す義務と正反対の内容になってしまう。
Dの「てはいけません」は禁止を表し、「返してはいけない」という文全体で意味が通らなくなる — 図書館の本を返すことを禁止する理由はどこにもない。
English explanation:
A is correct. "今日中に" ("by the end of today") establishes a clear deadline, and borrowing a library book comes with an obligation to return it so "なければなりません" (must / have to) is the only option that matches that obligation.
B ("てもいいです," may/is allowed to) weakens the sentence into optional permission, which directly contradicts the deadline the sentence already established.
C ("なくてもいいです," don't have to) means the opposite of what the deadline implies it would be saying there's no need to return the book at all.
D ("てはいけません," must not) reverses the meaning entirely, there's no reason in the sentence to prohibit returning a library book.
Notice what this actually does: it doesn't just confirm the right answer, it walks through the specific logical reason each wrong option fails in this sentence not as a general grammar rule, but as a contextual judgment. That's the skill the real exam actually tests, and it's exactly the skill a bare answer key can't teach you.
Why this matters more as you go up in level
At N5–N4, distractors are often simply wrong one option is obviously nonsensical, and a discrete answer key does less damage. By N3 and above, wrong options are deliberately built to be plausible: near-synonymous grammar patterns, subtly shifted nuances, one-word swaps that flip a sentence's meaning. At that point, a textbook telling you "the answer is A" is barely more useful than a coin flip you happened to get right it doesn't tell you whether you understood the sentence or just got lucky.
The honest limitation
This isn't a case against textbooks generally, a good textbook is still the right place to first learn a grammar rule in isolation, clearly and without the noise of four competing options. Where a textbook falls short is the second step: testing whether you can actually apply that rule under exam conditions, where the wrong answers are built specifically to catch a shallow understanding. That second step is where explanation depth stops being a nice-to-have and starts being the whole point.