Watering lightly in shorter intervals weakens the grass because the roots do not search out water, enabling the grass to better deal with extreme periods of weather. Watering after your lawn looks like it needs it will take much more water and effort to get it back to normal, because it has already been damaged. Your lawn, just like all organisms, is doing a balancing act. It has to resist diseases and insects that are attacking constantly. By watering in the evening, you extend the time your lawn sits damp. This gives disease pathogens (fungus) already in your lawn an ideal environment. Also, watering is what activates and circulates the weed control. Without it, you will not get good weed control and the lawn will be stressed.