There is a lot of debate on use of props. Some purists feel yoga with props reduces the value of yoga, while others believe it assists those unable to execute a pose properly. Are props only for sick people? May it also be used by healthy beginners to reach deeper into a pose? Can an advanced practitioner further one's practice with props?
When the Indian government roped in experts to draw a list of yoga practices that were getting swamped by patenting, they thumbed down props as unyogic since these do not find a mention in ancient treatises. So the confusion over props only got more compounded.
But schools based in classical yoga also use props. A good example would be the Iyengar school which uses ropes, chairs, wooden bricks, bolsters and cushions to reach a beginner or a very stiff person into a pose. It must also be remembered in olden times, warriors expanded their yogic practice with use of rope, cane, wooden rods and poles creating the exotic and powerful branch of yoga called mallakhamb which is gaining great popularity even abroad today.
So the use of props are not entirely a new feature of yoga. Once when two wrestlers from the Nizam of Hyderabad's court challenged a Deccan Peshwa's warriors to a bout, an -18-year-old named Balambhattdada Deodhar offered to fight them. He retreated to a village to prepare for the bout. It is said Lord Hanuman himself trained young Deodhar, using a tree as an equipment, teaching him to practise gusti (wrestling) on a pole, as with a partner. Mallakhamb is tracked back to this incident. Deodhar returned to shame the Nizam's team, restoring the challenged honor of his Peshwa who promptly hired him as his personal coach. So, use of props does have historical and mythical links.
Students do feel reassured by props and are willing to experiment with difficult poses. Here it is crucial to remember that most injuries happen when the muscles, at the fear of falling, contract and become taut instead of retaining their supple bounciness. Often a student, even when trying something mild or simple, will be in a state of muscular tension that will erupt later, while relaxing, as pain. All these initial hiccups are avoided with props.
Of course, there are some cumbersome props that become addictions, like padmasana (lotus pose) stools. I know people who carry these cumbersome props with them even while traveling, using them to hold up the knees while sitting cross-legged! As a beginner you will find it difficult to make a choice between a necessary prop and an unnecessary one, the latter often recommended by commercial schools. But basically, most props should be just good-old household items, like foot stools which can be used to expand your plough (halasana), the bolster to enhance deep breathing in a fish pose (matyasana) or a wall, to prop up a hand stand or a headstand. Anything beyond these are usually avoidable. Similarly, the very obese or very old may use a wall to grow in their shoulderstand ( sarvangasana). Wrist strength cannot be taken for granted. Some poses exert a lot of pressure on the wrists. It is ideal,in such cases, to strengthen the wrists by using props for some months and then dispensing them once confidence and strength is built up.
If you are advanced practitioner you may wish to invest in a gym ball, to challenge your balances in commonplace poses like boat (navkasana), bridge (setu), cat (marjari). Pilates, an innovation of yoga, uses belts. These are okay for a beginner, but unnecessary for an advanced practitioner. The latter have inner, muscular strength to withstand the drag of gravity which is tough on beginners.