Learn Hardcore Chinese: tā zhīsuǒyǐ duì lìshǐ yǒu xìngqù, shì yīnwèi yéye chángcháng gěi tā jiǎng gùshi. - The reason she’s interested in history is that her grandpa often tells her stories.

她tā　之所以zhīsuǒyǐ　对duì　历史lìshǐ　有yǒu　兴趣xìngqù，　是shì　因为yīnwèi　爷爷yéye　常常chángcháng　给gěi　她tā　讲jiǎng　故事gùshi。tā zhīsuǒyǐ duì lìshǐ yǒu xìngqù, shì yīnwèi yéye chángcháng gěi tā jiǎng gùshi.

The reason she’s interested in history is that her grandpa often tells her stories.

Breakdown of tā zhīsuǒyǐ duì lìshǐ yǒu xìngqù, shì yīnwèi yéye chángcháng gěi tā jiǎng gùshi.

Questions & Answers about tā zhīsuǒyǐ duì lìshǐ yǒu xìngqù, shì yīnwèi yéye chángcháng gěi tā jiǎng gùshi.

What does 之所以 mean here, and why do we need it?

之所以 is part of a set pattern “之所以……，是因为……”, which means “the reason why … is because …”.

之 here is a classical particle meaning “that” (as in “the thing that / the reason that”).
所以 here means “reason” or “cause” (related to the modern word 理由 “reason”).

Together, 之所以 doesn’t just mean “therefore” (like 所以 often does in modern Chinese). In this pattern it points forward to the reason that will be given after 是因为.

So:

她之所以对历史有兴趣 = “The reason she is interested in history…”
是因为爷爷常常给她讲故事 = “…is that her grandfather often told her stories.”

You could say simply:

她对历史有兴趣，因为爷爷常常给她讲故事。
That’s fine and more neutral. Using 之所以……是因为…… adds emphasis and makes it sound more explanatory or formal, like you are explicitly laying out cause and effect.

How does the whole pattern “之所以……，是因为……” work? Can I change its order or leave parts out?