Why did city walls have protruding towers every 50 meters?

Answer: To shoot attackers along the wall, not just out from it

To shoot attackers along the wall, not just out from it ✓ — Correct! A flat wall has a 'dead zone' — once attackers reach the base, defenders on top can't shoot down at them without leaning out. Protruding towers (called 马面 in China, mural towers in Europe) let archers fire ALONG the wall face, hitting attackers from the side. This is the ancestor of every later flanking-fire design, including star-fort bastions.

To give kings somewhere to live above the city — Wrong. These towers were military positions, not residences. Royalty lived in keeps inside the city, not in the perimeter towers — those were too exposed and too small.

To store ammunition closer to the fighting — Wrong. Ammunition was stored in arsenals near the gates, not in every tower. The towers existed for the geometric reason: covering the wall base with crossfire.