The first are those integrated into your AP infrastructure that provide initial estimations of WLAN coverage and real-time dynamic radio management. The second type run on laptops to assist in the on-site survey walk-through. The third are full-blown modeling applications that provide intensive WLAN coverage calculations.
These tools offer interfaces that let you run visualizations of your WLAN and design it for ubiquitous coverage. With visualization, you can virtually manipulate or relocate access points, sensors and antennas without having to physically move them by trial-and-error. Site-survey tools also let you test "what if" scenarios (the addition of access points, hardware failures, a swell in the number of users); identify and avoid sources of interference; and support a performance baseline for your WLAN.
You may decide not to perform a site survey if your initial WLAN deployment is small and you're building out your WLAN incrementally. But once that WLAN starts to expand, a site survey is inevitable.
If you're building a WLAN at a site with no existing wireless infrastructure, the big question is how many APs you'll need and where they should go. Depending on the AP's architecture--whether it's an autonomous AP or a thin AP with a WLAN switch--you may be able to hold off on buying them and use an AP estimation tool first.