Over-watering can be as detrimental to a plant as under-watering. How much water your garden needs will depend on the type of soil you have - sandy soil needs more deep and frequent watering than clay soil that holds water and is prone to waterlogging.
A further consideration is the humidity of the air - in most parts of the world this will vary not just with the seasons but also day to day. On a day when humidity is low, transpiration will increase.
A garden should not be watered uniformly. The amount of water each part of the garden needs will depend on factors such as sun and shade, plant density and elevation.
Appropriate watering is a qualitative as well as a quantative issue. The flow of water should be fine enough not to damage plants and steady enough not to cause run off from the soil. Some plants like water on their leaves, some plants do not. Some plants have deeper roots than others so will need to be watered less frequently. Modern garden tools offer a myriad of attachments and settings to give you control over how the water is delivered to your plants.
A watering can is a labour intensive way of watering a garden and may well be impractical if your garden is large. However, the watering can affords a degree of precision impossible to attain with a hose or even the most high tech of sprinklers. Watering with a can allows the gardener to cater for each plant's individual needs.
Fresh water is a precious resource. It is crucial that all gardeners use hoses and sprinklers responsibly.