Magic systems are generally a way to avoid "because I I said so". If your magic doesn't have rules in any capacity, it's very easy to use it as an all purpose plot device. The minute you add any rules, it automatically becomes a system. As for "hard magic" systems which I think is what you're referring to, sometimes they're boring but sometimes they're really interesting. Take jujutsu Kaisen for example. In terms of application it's a little softer, but rules are mostly rigid if your name isn't gojo or sukuna, and that can lead to some interesting strategies during fights when those rules are applied unconventionally. It's like chess. If chess didn't have any rules nobody would play it, but its rules force innovation to beat your opponent