Answer:
Cytokinin are made in greatest amounts in the roots. Simply put, they may be an indication that things are going well for the root and shoot growth may therefore commence. The principle roles of the root are to take in water and minerals from the soil, to anchor the plant and to store nutrients like starch that has been made by the leaves (the last in some but not all plants).
Since it's possible that salicylic acid acting as a hormone may be an indicator of an abundance of water, cytokinin may indicate that the plant is taking in more than enough minerals from the soil to just survive, so that it is OK to start growing more stems and leaves. In fact cytokinin inhibits senescence of leaves, and promotes new stem growth. Whether it needs a high level of salicylic acid also as indicator to give the green light to stem growth, is a good question...
It's also quit probable that even if the plant is taking in good amounts of water and minerals from the roots they would not make salicylic acid and cytokinin if the root were under duress for other reasons, say by infestation, flooding or dislodging. In other words any condition that plant is in where it might be good to wait to start growth until a better situation is achieved. It's possible cytokinin is still made under these circumstances but the minerals are instead store for a better day.
One last possibility is cytokinin alone indicates root prospering just as perhaps auxin alone indicates stem prospering. The reason for saying this is that cloning a single isolated plant cell into a full new plant, seems to require only cytokinin and auxin, not the additional salicylic and jasmonic acid (jasmonate is proposed in this present author's scheme as a sugar indicator with Auxin relegated to a role as a carbon dioxide and oxygen uptake indicator).
Mind you most scientist woulds consider this entire answer highly speculative.