What are the advantages of an (environmental) due diligence survey?

I'm going to assume you're referring to a construction or real estate project here.
performing an environmental due diligence survey would you give you a better understanding of what your future costs would look like in terms of the "environment" or location of the project.
this will in turn shed some light on what costs you need to consider such as future capex to maintain or transform the building/land/project to conform to regulatory laws (asbestos, fire hazards, soil problems...), or insurance considerations you need to take in to account.
the better you know the possibility of future costs, the better you'll be able to price your project or value the business you're acquiring with no surprises.
i know this is a general answer, but given the limited information, this is a top level view of the benefits of environmental due diligence.
for disclosure, i have not performed or been part of any environmental due diligence.
if you have more questions, or if you have follow up questions on this with more information, i'd be glad to help out.
-Nizar