I've used a JBJ Submariner in-tank UV - 24/7 in my planted discus tank for several years.
1. It will destroy ANY algae that you can manage to get free-floating into the water column;
2. When used properly, i.e. the correct wattage for size of tank, and slow enough water movement through the UV for optimum exposure, it will get rid of parasites and other potentially harmful pathogens;
3. Depends on your personal preference - the looks of it within the tank - I prefer a self-contained unit;
4. Answered above
It also helps to improve surface agitation, water circulation within the tank, oxygenation, water clarification & purification, to produce overall better water quality and conditions. Worth every dollar they cost.