There are a few reasons people recommend runoff. The main reason is a lot of growers provide very high levels of nutrients in their water, which starts to build up in the soil if you never get runoff out the bottom, causing nutrient burn and sometimes lockout or deficiencies. It can also cause the pH to go haywire. Additionally, if you don't give enough water at a time, the roots don't grow to the bottom, so they aren't utilizing all the space. Lastly, cannabis plants often seem to do well with wet/dry cycles, where they get a lot of water at once and then get a little time to dry out. That being said, you can try to keep a constant moisture.
The main thing to remember is that cannabis roots have very high oxygen needs. If the soil doesn't get time to dry out, or if it gets waterlogged, then the roots may likely aren't getting enough oxygen.
If you're not giving nutrients in the water (for example if you're growing in organic soil), you probably don't want to get a whole lot of extra runoff (maybe just a few drops to make sure the water got all the way to the bottom) because giving lots of extra plain water will start slowly leaching all the nutrients from your soil.
The main idea behind watering frequency, and all the different methods of providing water to your plants, is to try to make sure your roots are always getting a lot of both oxygen and water.
If the roots don't get enough air, the leaves get fat and full of water, causing the leaves to droop. Drooping from overwatering slows the plant down (which is why chronically overwatered plants stay really tiny), but unless it's really bad you usually won't get long-term damage to the leaves.
If the roots don't get enough water, the leaves wither away because they have no water to fill them, which also causes drooping though leaves appear more limp. Since they both cause drooping it can be hard to tell what's happening. To add another wrench, if a plant dries out a LOT, sometimes you can water it and it will still get droopier for the next day or two because the leaves were damaged, which can be confusing because it almost seems like giving water caused it to get worse.
Any method to accomplish a good water/oxygen ratio will work. That's how you get hydroponics where roots are sitting in the water -- they're constantly bubbling oxygen and dissolving it into the water, or in aeroponics where the roots literally hang in air, and get their nutrient water from a mist. One thing to remember is that plants are not able to drink as much when it's humid (the way the plant gets water is it evaporates water from the leaves, sucking up water from the roots like a straw, which is not effective in high humidity). They also tend to get droopier when it's hot because water can hold less oxygen at higher temperatures (that's also why it's important to control the temperature in hydroponics).
