Question

why it is not advisable to give excess water to water plant

Answer 1

That's one reason that set-it-and-forget-it automatic sprinkler systems — which often can be seen watering in the rain — not only waste water but also are bad for the grass and other plants. Symptoms caused by too much water, such as wilting branch tips on trees, can look the same as those of too-dry soil

Answer 2

It is not advisable to give excess water to plants because excessive watering plants may harm the crops.Water is the life of plant and must be supplied in proper quantity. Most of the soils receive water through the rain also water is added by the way of irrigation from well, tank or canal etc. Too much water may suffocate the plant roots and to little may not be able to sustain the plants. Excess water is 
(a) harmful to crops and soils, 
(b) wastage costly input