For sellers, there is no point to raise any flag. It's for their least interest to find anything, pending mitigation, tons of money, lost of time... For them to hire a pro, who then represent sellers' interest, should 大事化小 小事化了, cheap and quick remedy.
For buyers, you want yourself protected. Environmental issues could be big money. So if you hire professional, they'll represent your interest. Phase 1 usually doesn't do anything, Phase 2 not much either. At least you should know the situation, and/or to use that for bargain power.
Same thing for lease, you want a clean starting point, which mutually agree. Otherwise when you move out, owners will bite you, which happened in my case. At that time it becomes your responsibility to prove the site is clean. They might even say you are the reason they couldn't lease the piece to next guy, which we all know it could vacate for 1 year easy.
