All plants naturally possess hormones, including the growth hormone auxin, which is produced in immature parts of the plant, where growth is necessary. Synthetic auxins are used in hormone rooting powders and gels to replicate natural growth conditions in plants, and encourage roots to form. Hormone rooting powders often also contain cytokinins (another plant growth hormone), fungicides and other chemicals, which reduce the risk of the plants succumbing to fungal infections.
Rooting hormones increase the chance of your cuttings taking root. What’s more, the root will usually develop quickly and be stronger than when plant-rooting hormones aren’t used. While many plants root freely on their own (see below), using a root hormone makes the task of propagating ‘difficult’ plants much easier.