7-Zip’s main advantages are that it’s free, it will open most archive formats and does a better job of creating new compressed file archives than its competitors. As well as supporting its own 7z file format, it can compress industry standard zip files further than commercial rivals like WinZip can, so the resultant archive is smaller but still compatible. This is particularly useful if you need to send documents by email or offer them for download from a web site as it will help to reduce your bandwidth usage.
Its format support is extensive; whilst it will only be able to create archives in Zip, GZip, BZip2 and 7z formats, it is able to open a much greater variety including RAR, and it will also let you see inside ISO image files and Debian packages, amongst others.
The interface isn’t great, but like most archiving programs it integrates well with Windows Explorer so you probably won’t need to use it much. And it’s free, open source, and works well with both 32-bit and 64-bit Windows.