It is always in the best interests of insurance companies to limit their losses on settlements, and they typically work with repair shops that provide estimates based on the best interests of the insurer.  The insurance company may also leave things out of your settlement such as transfer fees and sales taxes and they are not legally obligated to inform you of all of the entitlements available to you.
An attorney that specializes in total vehicle losses will inform you of your legal rights and help you get a fair settlement.  They will review the documents of your claim, including the market value report and repair estimate as well as other applicable taxes and transfer costs.  An attorney can also negotiate the settlement amount on your behalf to ensure you get a fair total loss car insurance settlement.