Is Arduino useful? Which type of Project is better For?

First learn DC theory. Then learn AC theory. Then learn how semiconductors work. Then op-amps. Then digital electronics. If what you want to do is still too complicated to do with that knowledge, then go look at microcontrollers and microprocessors. If you do it in the reverse order, you won't have any understanding of electronics, you'll just know how to hook up wires and write code. Of course if you don't care about actually learning anything then do what you want, but if you want real knowledge of electronics, then do it in the order stated above. Microcontrollers can be fun to play with but they won't teach you much of anything about actual electronics, you'll always be behind the curve and feeling like you're missing out.