A $1,000,000 Inventor's Insurance policy is designed to go up against larger corporations that might infringe your idea, as opposed to smaller companies.
A $1,000,000 policy is enough to enforce your patent against a large company and survive them trying to invalidate your patent.
The typical patent lawsuit begins with a complaint that you make against the infringer in Federal Court. There will be some back-and-forth, and the infringer will often make a counter-claim that your patent is not valid.
If the infringer succeeds in invalidating your patent, your lawsuit gets thrown out and they get away free. If you succeed in defending your patent, your patent is 'gold plated' and the infringer has a very steep road to climb to win. Usually, there is a settlement offer either during the invalidity proceedings or immediately after.
A typical invalidity claim will be an Inter Partes Review proceeding at the USPTO. The average cost for you to defend an IPR is $300-500K, which is covered by your Inventor's Insurance.