I've seen them (bones) most commonly used for divination purposes. When you cast the bones, the positions in which they fall, etc can be used to determine the outcome of your question or reading.I personally use bones and and other such related materials to connect to certain energies or spirits when doing magical/ritual workings. I've also heard of bones used for housing spirits and thoughtforms.
For example, I've a relative that uses a deer skull as a housing vessel for a familiar spirit. Skulls are wonderful for spirit housing and can be cleansed and prayed over to invite familiar and totem spirits to your space. Bones can be a direct connection to the spirit of the animal from which they are taken. If the spirit of the animal (the owner of the bones) does not wish to work with you, you can invite another similar spirit (one of the same species) to reside in the spirit.
To entice a spirit into the bone vessel, you need to cleanse it and prepare it as a house for the spirit. I personally bleach by bones with a 25, 25, 50 mixture. 25% peroxide, 25% bleach, and 50% water. So, if I do 1 cup of peroxide and 1 cup of bleach, I would use 2 cups of water, as an example.
Once the bones are bleached and cleaned, you can them allow them to dry. Soak them in salt water, wipe them off, and allow them to once again dry. Then, anoint them with an oil of your choice (I usually use cedar or sandalwood, just as a personal preference.) Then I pray and chant over the the bones (usually a skull) and ask the spirit to join me and work with me as a guide.
I have a special altar upon which the bones are placed and to it I add candles and herbs associated with the spirit. For example, if I were using a deer skull, I would have cedar woodcandles, acorns, dried grass pods, and other items that the creature enjoyed in life.
It can take time for the spirit to come and reside in the vessel, but you will know once they do.