Partial content means just that: the web client asked for part of the content, not the whole thing. Besides being used by download managers (think pause/resume download as well as a speed trick) it could also be used by clients to cap downloads (eg limit to 10k in the event the url is to an image file instead of an html document).
You’d have to ask someone who wrote the client software why it would ask for partial content, but the HTTP 1.1 Spec does say if a client specifies a range of bytes to download, the status code will be 206 instead of 200:
RFC 2616 HTTP/1.1 14.35 Range
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