You need to train with weights to build and maintain muscle for a sculpted physique. You may maintain muscle without lifting weights in some situations, but it's always optimal to include weight training in your program when getting lean.
The reason you have to train with weights to build muscle is because the body grows muscle as a direct response to overburden. If the body realize that you're lifting loads that are too heavy, it'll respond by growing bigger muscles to help with your survival.
Therefore, whenever you hit the gym to lift weights your aim should be to send the message to your body that you need it to grow bigger muscles so you can be stronger. Achieving this requires a very effective weight training program that comprises of the right weight lifting exercises, correct rep
and set scheme and heavy weights to create stress and damage to existing muscle tissues. Damaging the muscle during a workout is your signal for growth. Each time the muscle is torn down it'll grow bigger and stronger thus the targeted muscle will look bigger on your body after a period of time of consistent breakdown and recovery.
Getting significantly bigger muscles to appear on your body requires sticking to the process of damaging the muscles in each workout session, and the way to achieve is to constantly increase the size of the weights you're lifting which is known as progressive resistance.
Resting the muscle is also very important when you're trying to build muscle. The muscle repair and grow bigger when they're in a resting state, not while you're working them out. Any muscle group that gets worked to the point of great damage must not be worked again until after about 3 days, at which time they should be fully repaired, stronger and a bit bigger in size. If you work the muscle again before it recover properly you'll be overtraining.