Did you know that water drainage is extremely important to the health of your garden? If more water pools in your gardens than your actual pool, your plants will drown. If the water leaks out of the planter faster than you can put it in, your vegetables will be dryer than the Sahara desert. There is definitely a happy medium to both though.
What your are looking for is water retention but not too much. You'll want the soil to remain moist long after you have watered your garden, but keep in mind that you'll want to ensure that your soil level is the appropriate depth so that even if water sits on the bottom of the planter, the roots will not be submerged in the water.
There are risks when the roots of a plant are fully submerged in water. Because plants need not only water, but oxygen to live, roots fully submerged in water will not bring in the required amount of oxygen for the plant to thrive, and it will suffer. As well, sitting water is a hot bed for fungus and bacteria which can feed on the roots of your veggies and kill them.
By ensuring proper drainage in your garden, your plants will have a long and healthy life!