When is transpilePackages necessary?

[sullen patrol]

I'm setting up a new turborepo monorepo with some nextjs 14 apps.

I have apps/web and apps/docs as test applications and they both use a packages/ui component library. When I run a dev server, I see no difference using transpilePackages: ['@repo/ui'] or not. I guess I'm not understanding what it's doing and why/when I need it. Wouldn't those internal packages be included anyway in dev/builds because I'm importing their code?

[silk flax]

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[lethal venture]

If you look here.
https://nextjs.org/docs/app/api-reference/next-config-js/transpilePackages
It states that Next JS will automatically transpile and bundle packages from monorepos.
The only time I had to use transpilePackages is when I was using the crypto package. https://github.com/brix/crypto-js/issues/415#issuecomment-1837938710

[sullen patrol]

I did read that, but I guess I'm not clear on why it's needed. I think it's because I'm importing files from another internal package that would not be "available" if I built this project because unlike node_modules/depedencies, I'd have to tell nextjs to bundle them

when I run dev mode, they're available because my whole monorepo is avail, but if I were to deploy the nextjs app, those internal modules wouldn't be included by default?