Why download AND upload?

This might be a stupid question, but after having some problems with synchronizing I finally decided to start again from scratch and deleted the zotero folder on my computer at home to synchronize everything new from the server (including file-synching from webDav server). I´m not a computer expert, but I thought that in this case (everything removed from the client computer) there are only files that can be downloaded (from the server). Why are there also files that are uploaded? At least this is what the synching progress bar tells me.

Thank you (for an answer and for Zotero),
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