Hi, does pact test helps only when both consumer and provider use it? what if I want to test my system’s contract with 3rd party system, but the other side doesn’t use pact?

There's a whole FAQ about that 😉 https://docs.pact.io/getting_started/what_is_pact_good_for#what-is-it-not-good-for But, short answer, yes (that doesn't give you the best value).

I see. Thanks. Then I’ll reconsider using it internally where both sides are under our control.

I think we have an AMA on that: https://docs.pact.io/help/amas

I usually still write a pact test even if I'm not going to verify the contract. However, when you do that, pact is no better than any other mocking tool (and you get very little confidence out of the test, since you are marking your own exam). The reason I do it is so that I have the pact file in case I ever can verify it.