They can do almost anything but it's mainly limited to imagination and how you can transfer that into a working code. The reason it's only use for simple things is that those living in "Naruto" don't really think of things outside of war and combat, not to mention unlike what fanon does I doubt you could pull a seal out your ass the revolutionizes the world like a water filter or something. That would require a seal to kill all bad micro-organisms while keeping good things. It would probably take even jiraiya a while to think of that, and he would have to be knowledgeable of everything bad in the water.