There is no reason.

What it all boils down to is how deep do you want your plant roots to be able to grow healthily?

For many plants it doesn't make much difference beyond 6". If you have a plant that will grow acceptably for you in 6" of soil then you can fill the rest with cement for all it matters.

If you have a plant that does best with 12" of aerated, moisture/nutrient retentive soil then you probably don't want to use bottles/cement/broken pots/rocks or anything other than top quality potting mix for 12".

Can you use a cheap mix on the bottom and a premium mix on top? Sure.

The question is will your chosen plants do well in the great top zone? Will they get rot from the less aerated bottom area?

I mean in most cases plants will do fine with yard dirt in a pot. It is a PITA to water for us, but the plants will do OK. So, why not just use yard dirt in pots?

Main reason is plant vitality. The question is how far are you willing to go in order to get the last iota of performance from your plants?

Somewhere along the line there is a law of diminishing returns that comes into play, but where we draw the line is an individual matter.

Some do grow in yard dirt filled pots and are happy. Others use Miracle Grow potting mix from top to bottom and are happy. Others use MG potting mix for 8" and then garbage filler for the rest and are happy. Others swear they must use their homemade, custom super mix for the entire pot in order to get results that make them happy. Some say peat is great, others say it is evil.

Bottom line is plants will do well in damn near whatever you put into the pot. To go from 'doing well' to 'Wow! these are doing fantastic!!' you pretty much have to settle upon a way of doing things and then convince yourself it really is better than everything else. ;-)

And truthfully, convincing yourself that you have stumbled upon a better way to do things is part of the fun. It may not be true, but gardening isn't about truth, it is about having a good time.

I found a couple of styrofoam heads, the kind you display a wig on at the Goodwill for 49 cents each, and very clean. I put just one in each of my large pots, and then filled with MG soil.
It really worked great. I will be using the pots for annuals, and short root veggies ~ Judy

Karen - there are conditions in which adding something to the bottom of a container might be helpful. If you have a smallish plant or planting and a container you feel is too large, it might be appropriate to add filler to the container to reduce o/a soil volume and reduce the likelihood that soil will remain wet too long. Of course, this can be accomplished by only partially filling the container with soil, but most people want their containers to at least appear full of soil. You can still fill the container with soil and add a wick, which will remove most of the extra water, as long as your soil is at least close to appropriate for use in containers. Later, when the plant(ing) matures, you can remove the wick & the plant will have the entire container from which to draw additional water & nutrients. Soil volume is important to plant size/health. You need only look to the dwarfing effect of small containers in bonsai culture for a prime example.

The reason that it's a poor choice to mix different soils in the same container is one will always be too wet or too dry, greatly increasing the likelihood of root rot issues. People think drainage layers work because they think they help the soil to drain, when in fact, their value lies in the fact that the total volume of water held in soil is reduced commensurately with the o/a volume of soil. Drainage layers actually move the saturated layer of soil higher in the pot, not a good thing, but often plants tolerate it because of the reduced volume of soil. Why not build a soil that we can use from top to bottom in containers? - one that drains well & provides adequate aeration for the planting?

I would never suggest you use garden/top soil, fine sand, or compost in containers. All kinds of growers post here & base their comments on their garden (real garden, not container garden) experience, or what they think should work, or simply say "It works for me" or "I've been doing it this way forever and ..." They never say it's better because ...

The physical make up of a very good container soil will contain from 60-75% total porosity, with half of that being air porosity. This means 30% of the soil should be filled with air spaces immediately after you saturate the soil and it has just stopped draining. This structure will provide the best medium for roots to grow with maximum vitality, which translates to best plant vitality.

When you add sand, garden/topsoil, or compost to your container mixes, you destroy the aeration, create drainage issues, and place yourself under an unnecessary handicap from the outset.

Al

What Al says. The easiest mistake to make (in my own wretched experience) is to put "garden soil" or "topsoil" at the bottom. This clogs up the drainage and will kill something like a rose (which hates wet feet) in no time. Something tough, like ivy, say, might just drill down into it successfully, but you can't count on that.