The box kits are nice, but too often the materials are not needed or appropriate for the flies of the area in which the tier lives. Seek that advise of a fly shop or competent tier. Many flies are representative of the food that fish eat. In other cases, the flies are meant to deceive the fish. You are constantly learning about the food sources and trying to imitate them with the flies you tie and the way you fish.
Different fish feed on different food sources.
Ocean game fish don't eat bugs, for the most part. River and lake fish survive on bugs and other things, like small fish, frogs and leeches. But buying flies can be very expensive. And, there is an art to choosing the right fly to buy. Making them is just a lot of fun.