Garden soil will tend to pack down hard in a pot over time, perhaps less so if it is full of organic matter/compost, whereas potting soil is going to remain aerated and porous. It has additives like perlite or vermiculite to ensure this.
plants require air to the rootsystem as much as they require water.
The rocks in the bottom don't really do much except mean you need less soil to fill the pot. It's the holes in the bottom that do the draining.
Drainage is a must for all
plants except bog type
plants and those that have evolved in wet conditions. Generally, the common herbs are not among them.