Because utilizing a multitude of tools may seem insane, the main reason for this is due to the fact of cross-referencing information and double-checking things. This is said true for pinging too! Fping is another utility that will assist in pinging systems and attempting to return information regarding if they are responding or not.
nmap is the defacto standard when it comes to port and service discovery and some version information gathering regarding the ports. This document will also be extended to cover the NSE (Nmap scripting engine) which can do a lot more than the standard feature set of nmap.
This process helps in either testing your firewall, and also discovery of the system 1 hop passed the main firewall in your network. Understanding a firewall ACL may also help you better understand the security posture and bypass security devices!
SNMP may be utilized to gather information about a network, its topology and systems regarding (potentially) user names, versions and other information that can be utilized effectively in an attack. This information can be from up time build numbers and more! Adding this information to a penetration test can also help you select user accounts to brute force, and systems you may be well versed to exploit.