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There is less point these days with growing hard drives, FTP uploads and broadband but still as a previous poster noted it is a good way to archive old files.
 
Sort of a subset of the "because a single file is easier than multiple files" reason above... Because I can create zip files with the drawing file, the fonts, x-reffed drawings, ctb print schemes, and anything else associated with my drawing files all in one folder per drawing sheet.

Makes transmitting drawings a much easier and cleaner process. All files associated with a single drawing are contained in individual zips.
 
Some ISP's also cap the size of emails you can send – that's what ours does so the ability to shrink a large file down by zipping it does come in useful.

And some ISPs, to avoid the spread of viruses, limit the type of files you can send to them. For example, I had a buddy once where I had to ZIP up all executable files and PPT files (that might have auto run) or they would get rejected. They didn't want anything that could be accidentally clicked on and run and potentially spread nasty things around.

SCR
 
I don't know about zipping, but I know that the compression rates of tar and gunzip save my colleagues and I a ton of space.
 
I don't know if it's this way still, but back a few years ago I remember if I sent an EPS file through email it would usually end up getting corrupted during the transfer. Zipping the file first would eliminate that problem.
 
When I compress files using OS X's built in "Compress" feature (right-click > Compress) I get almost no reduction in size. Am I doing something wrong here or is this not the same thing...?
 
When I compress files using OS X's built in "Compress" feature (right-click > Compress) I get almost no reduction in size. Am I doing something wrong here or is this not the same thing...?

The amount of compression possible depends a lot on the type of file you're trying to compress. For example JPEG files are already compressed internally, so you will notice little change in size by zipping them. Text files, particularly very large text files, will compress extremely well.
 
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