The most important thing with containers is consistency and frequency.
Containers lose water faster, though the container, as well as through the plant's usage. There is just more surface area to transpire the moisture.
It is harder to thoroughly wet the entire soil mass in the container, especially if the soil has more drainage than the ground. The drainage characteristics of your potting mix may allow the waqter to flow down and out before it thoroughly wets the potting soil. If you apply the water via a point source, such as drip, it may not spread laterally to the entire root ball.
The beauty of the self-watering idea is that it captures more of the water applied to the pot (no holes at the bottom) and allows the water to be used more on an as needed basis, as the soil dries out and the water is wicked up to compensate. It also allows time for the moisture to wick through the entire root ball.
That doesn't mean it is the only way to deal with the issues, or solve the "problems" of container growing. It is a solution. When you understand the issues, yo are free to address them in a variety of ways. If someone has already shown how to solve them that doesn't mean that it isn't a solution that should be used. When you have a good solution, why re-invent the wheel so to speak.