Both of the cauliflower plants in the photo were the same size when I planted them a month ago. The transplants got the same sun and rain, but the normal one on the left was planted in a commercial potting soil, whereas big brother was inserted into custom soil. Why bother to mix soil?
The same reason I mix my own cocktails. I rarely meet a blend I like, whether it's dirt or drink, and I believe I can do better. The difference is that the taste of a Manhattan is subjective, whereas plants either grow better, or they don't. The big cauliflower doesn't fret about what kind of rye whiskey to use, but it looks happy where it is.
There are good commercial soils, but the bags are expensive, and contain vague "composted forest products" and wetting agents, which are similar to the chemical compounds that make disposable diapers absorbent. The dirt blend I make doesn't contain anything that artificial, and is ideal for pots, containers, or raised beds, providing a good balance of nutrients, friability (it crumbles easily) and drainage.
The sand is for drainage, and cheap topsoil often has sand, too. Perlite work better, but with heavy watering the white stuff will float to the top of the pot and become unsightly. Vermiculite is more expensive, but drains best of all, and stays mostly in place.
Irrigation The Texas drought may go on next year, so an irrigation system is an increasingly smart idea. I tested a $20 Mister Landscaper water timer for a year, and it was trouble-free, despite exposure to rain, brutal sun, and ants. I used it with potted herbs, which required a Mister Landscaper Patio & Potted Plant easy watering kit for another $10, but it was simple to hook up the dripper stakes and hoses.
The system took a little fun tweaking, and it doesn't provide enough water pressure to allow too many drippers on one hose, but every plant thrived through the August inferno, and they're still growing despite the inevitable winter-time garden neglect.