Drainage holes aren't a necessity for happy houseplants.
Drainage holes aren't a necessity for houseplants -- but they make caring for them easier. Drainage holes allow excess water to escape --- so if you add too much water at one time, it seeps out the holes, and you can empty out the drip dray so the plant doesn't drown. But if you're careful and don't get into a situation where you're not adding too much water at any one time, then the holes aren't necessary.
As to why we grow plants in containers without drainage holes, we try to offer solutions for all of our retailer partners. Some outlets (especially grocery stores) won't allow plants in pots with drainage holes into their stores because they don't want a mess in the store as their staff maintains the plants.