Before the appearance of Rootrainers, growers had little opportunity to experiment and compare the merits of different depths of plant pots and in many cases they chose to downsize.
Air pruning of the root (the process whereby roots stop growing downwards when they meet air) occurs more quickly with a smaller pot. This process is not dissimilar to normal pruning that produces a bushier plant. In the case of roots, the air pruning encourages lateral growth and thus develops a stronger root system.
Some growers claim the quicker a plant is in its final growing habitat, the better. Plant too deep and the roots may be below the best top soil and be inhibited by poorer soil. There may also be a better balance between root development and top growth.
However, deep plants may be stronger and more mature, more resistant to disease and environmental changes. Under-bench heating will promote even quicker root growth, without plants having their top growth inhibited.
Deep pots enable growers to plant out earlier and to take advantage of well prepared beds, and good top soil. Growers will find they have different answers and Rootrainers can meet these different needs.
Basically, fast growing root structures, which is true of most vegetables grown in deeply well prepared beds, prefer deep pots, particularly for early growers. Plants that have to over-winter, like perennials may also benefit from deep pots. But annuals that go out after the frosts, into warm, shallow flower beds, and plants required for pots and hanging baskets require a shorter pot.
Most cuttings benefit from a shorter pot to encourage early air pruning; likewise, plants with slow root growth. Some such plants may need to be deeper before they go on out. They can be started in deeper pots and left, or they can be started in smaller pots for early strength and potted on into larger pots if required. This is a particularly easy process with Rootrainers because the pots are identical apart from the depth, meaning that the plant is not disturbed at all.
The ultimate choice is in the hand of the grower which is perhaps what makes plant growing so interesting.
With Rootrainers, instead of "pots" we talk about "cells", where each cell grows a single plant. The shape of the cells is designed to generate a far stronger root system than conventional plant pots. However the choice of size is no different whether you are dealing with a Rootrainer cell or any other plant pot.