By far, the biggest benefit to hand watering is the very low up-front cost. There are no parts to buy, no failing components to worry about - just a hose, a faucet, and your time in the garden.
Another huge advantage of hand watering is that while you are watering, you can look for signs of disease and pests on each plant. Powdery mildew, spider mites, aphids, and many more problems are much easier to treat when noticed early.
The last advantage that hand watering has over an irrigation system is that you can easily adjust how much water an individual plant gets from day to day. If the plant is drooping due to heat, give it some extra water. If the plant is getting too much water, simply water less.