Is it necessary to get a bone graft when having a molar pulled?

Not always. You should discuss the reasons with your dentist or oral surgeon.
No. If there is no extensive bone injury or damages, the granulation tissue will replace the clot and bone will be regenerate without a problem. Must keep the area clean to prevent infection to maximized the bone fill. Good periodontal health is the key to optimal healing.
No. Only if you want to preserve the amount of bone for later use. I would recommend grafting if you want to place an implant later.