This pattern shows what the speaker hopes will happen or thinks would be good. With といい, it often carries a wishful tone, like hoping for good weather or a positive outcome. With たらいい, it can also be a suggestion, especially in questions like どうしたらいい？ (What should I do?). Note that たらいい is more about a condition: if that happens, it's good. Both are common in conversation, but たらいい is more casual and direct, while といい is slightly more wishful and often used when you can't control the outcome, like hoping it won't rain. You can also use といいな for a softer, more personal hope, but stick to the basics for now. In writing, both are fine, but といい is more common in written wishes.