The answer is simple... Because it's better!
Drip irrigation became popular when the topic of "water conservation" became the "new" concern. It's not ideal for all situations, but is definitely the best option for flowerbeds. Let's review how the a drip system works.
On a normal system, water is released from the sprinkler valve and then goes to the sprinkler heads that are attached to that valve. But when you convert to drip irrigation, the water goes through the drip line (hose) instead of the sprinkler heads. A drip hose is usually around 1/2" in diameter with emitter holes every few inches that slowly release water. The drip hose should be right at or just under the top surface. Being visible is ok and normal.
The key so success in this system is that is "slowly releases water". This Allows the plants as well as the soil to absorb the water. There are a variety of soils and they all have different maximum absorption rates. The reason why you have puddles in your yard when it rains a lot is because the soil can't absorb it all at once. When you use a sprinkler system has spray heads, you will usually see water running down the sidewalks. This is because the grass/soil couldn't absorb all of the water and so the rest is just run off or waste.
A drip system is used to slowly release water, but this means you must run the system longer so that your plants get enough water. This doesn't mean you are using more water, in fact you are probably using a lot less, it's just coming out slower than what comes out of your regular sprinkler spray heads.
Conclusion: A drip system has many benefits including, less water usage, which equals a lower water bill, more efficient watering since water is dripping directing by the plant roots. Better freeze protection since the drip hose is more flexible than standard pvc, and lastly, since you can control where you want the drip hose to be located, you can put it directly by your plants/roots and therefore will not have other areas of your flowerbed being watered that don't need it. What does this mean? No more weeds growing everywhere in your flowerbed!