In which situation is it purposeful to use an active lane change assist system?

You answered correctly. Active lane change assist systems are designed to help drivers safely change lanes by monitoring surrounding traffic and assisting or warning if a lane change is safe. These systems work best on motorways and motorway-like roads because such roads have clearly marked lanes, consistent traffic flow, and higher speeds where lane changes are frequent. On roads without markings, these systems cannot reliably detect lane boundaries, so their function is limited or not available. Always use such systems in the environments they are designed for—mainly motorways and similar roads—to enhance safety and comfort.