There are many enemies in the universe so Star Fleet is important.Yes, but they're not enemies like Nazi Germany was an enemy, or like Al Qaeda was an enemy, or even like Bonnie and Clyde were enemies. They're enemies like a hornet's nest is an enemy. (If you're really cynical, they're enemies like the Protoss are an enemy, seen from the perspective of an American playing Starcraft. They exist to make your universe more fun.)
Think of how far ahead of the Klingon Empire the Federation is. They routinely have a single man with delusions of grandeur outwit the entire Klingon race and nobody thinks this is odd. We know that in the logic of the Trek universe if it's the entire Klingon High Command personally at the helm of 100 warbirds versus Wesley Crusher and his high school science project that Wesley is destined to win, right? You don't even need to see the universe from the outside to come to that conclusion. Ask a Ferengi to handicap that contest. He'd tell you "Rules of Acquisition #47: never, ever, ever bet against the humans. There's no profit in it. Ever."
Think of some power multiplier, X, for demonstrating the geometric mean between civilization's level of ability to effect their wishes on the cosmos. Multiplying the Aztecs by Xs gets you the colonial Spaniards. Multiplying the Spaniards by X gets you the US. Multiplying the US by X gets you the lowest starship captain ever depicted in the series. Multiplying them by X gets you to the Klingons. The Federation is still several powers of X beyond them.
This implies that the Federation could probably wipe out everybody else with a for loop, and probably consider that low-intensity conflict resolution. But that wouldn't be fun. Why hit the Borg with the Lance of Judgment when there are still so many stories to tell about how Picard proved that passion and ingenuity beat raw technological dominance? The Federation loves such stories as only a technological superpower can.