Used by companies large and small, from Google Desktop's installer, DivX Player, to your next door neighbor's pet project, PECompact has been a leader in the Windows executable compression industry for over a decade. You'll find no other compressor that offers the features of PECompact. It may be you don't need or want what PECompact has, but if you are one of those that does -- then PECompact is really the only practical option.
Software vendors will want to compress their Windows executables for a wide array of reasons, but the usual reason is to help protect the code and data inside. Compression inherently does this, though it is no fool-proof solution. PECompact is technically a Windows executable compressor only, but plug-ins are available for it that offer protection against reverse-engineering.