Automated workers have been dumb since they released Civ V. They remove useful features like jungle (for a university science bonus, or Hiawatha's perks), or marshes (Netherlands), where a human mind would consider leaving those once-gone-forever-gone features untouched for a bonus later on. The automated workers also flee too early from enemy units and barbarians, where a manually controlled worker could sometimes stay close to, but not capturable by, an enemy and then finish a tile improvement in stead of leaving half work.
A manually controlled worker has another benefit: they can be used to alert you when enemies are near in the fog of war (you can't see the enemy). An automated worker does this when they start running towards a military unit or a city for protection. A manually controlled worker stops doing what they were doing before the task was completed. This tells you an enemy unit is near (don't know what radius they sense, but it's like 4 hexes away (maybe more!), which is a lot!). This way, you can sense barbs, or if no barbs could possibly be near, an attack coming! Not easy to use, but very handy when you've gone to war with another civ, but don't know if they're coming for you or from where.