When doing any sort of preliminary Health Check for a Wi-Fi design, or when troubleshooting a problem, it is useful to get an initial overview of the Wi-Fi environment. 

A quick scout with a hand-held tester will give an initial indication of: Channel usage (both Wi-Fi and non-Wi-Fi) - see sample histogram. Levels of same (co-)channel and adjacent channel interference. Types of interferer (e.g. RF used for motion detection).

A hand-held tester focuses on high-level issues quickly, so it is particularly suited to diagnostics.

It's great for finding APs and client devices, and it also provides basic quality and capability information about them. It has specific diagnostics to test both Wi-Fi connections and directly-connected Ethernet sockets.and it can even be used to check range/roaming and throughput behaviour although we would prefer to use actual client devices to do these if possible if this is for a design.