At the level of dollars being discussed, there's a strategic benefit to engaging in negotiations just to force the counter-party to raise their bid or force concessions. For example, it's suspected that Google famously did this against Verizon for the FCC Spectrum licensing [1].

So take Walmart: If they believed there's a 10% chance that Amazon might join the fray (to target younger demographics), then there's a good game-theoretic argument that Walmart should be at the negotiating table too.

[1] https://www.cnet.com/news/spectrum-auction-google-wins-by-lo...