GIS allows you to overlay information, such as overlaying a flood map over the sales to see which were affected by hazardous flood zones. Mapping products are driven by consumer demand and are thus low priced and easy to use. GIS vendors focus on customized government and business applications. Few customized products have been developed for real estate. From Bruce Weber's chapter referring to a Gilbert Castle article.
Real estate is information arbitrage.
Too many real estate analysis reports of the 1970s and 1980s could be described as their unifying theme the binding that held different pages together. Both software and data must be considered. The data may be more expensive than the software. Schott's chapter has information on software. The principal users are government agencies such as the Bureau of Land Management and assessors offices.