Maybe I phrased that badly. You would have set kagi as your default search engine, but sometimes you want to search something on a specific site only. With bangs, kagi will redirect you to another search engine instead of running the query itself. If you append !w to your query, you will search on Wikipedia instead. If you append !g, your query is redirected to Google. There is no advantage over searching on Google directly, but it is much faster than going to the Google page if kagi is your default.
And I meant that the !g bang works on ddg as well, though there probably is a ddg bang on kagi too.
