Being able to solve NP hard optimization problems would enable progress in many areas of science and technology. For example it would allow us to find poly-sized Lean proofs for theorems efficiently, since proof verification can be done in polynomial time.
It would also be amusing to annihilate nearly six decades of proofs that assume P!=NP.
Could also break the basic principles underlying most encryption approaches. I would rather have my bank account not stolen and internet working
Even if P=NP it doesn't mean that the P approach will be better than the heuristic approach we already do today.
Leans proof checker is not polynomial time, unfortunately. It is super exponential. Basically, because it can verify the result of any function it can prove to be total.