It's required by most licences (open-source or not) for the third-party softwares/software components they use/have modified

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And since in most cases, you have to reprint the licence itself in full, it gets pretty large pretty fast

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Traditionally with open source softwares, these are distributed along the software/software sources in a dedicated text file (usually named COPYING), with just a mention of the software used/licence header visible in the program itself (with detailed info on how/where to view the full text of the licence) [or not, depending on the licence in question].