1. Allows low-level capture of network traffic. Most UNIX-based sniffers use this library. Misconception: You must have root privileges to run libpcap-based programs. This is a common problem when script-kiddies try to run programs based upon this library: they don't know they must run under root, and the scripts themselves rarely give instructional error messages as to what exactly is wrong. Point: Most Windows packet-sniffers are based upon a variation of the PCAUSA driver. This includes "WinDump", a libpcap implementation for Windows. From Hacking-Lexicon

2. Libpcap provides a portable framework for low-level network monitoring. Libpcap can provide network statistics collection, security monitoring and network debugging. Since almost every system vendor provides a different interface for packet capture, the libpcap authors created this system-independent API to ease in porting and to alleviate the need for several system-dependent packet capture modules in each application. Install libpcap if you need to do low-level network traffic monitoring on your network. From Redhat 8.0 RPM