There are several reasons that you may need to get a professional real estate appraisal.  The primary use for an appraisal is for the purchase of a home, the largest, single investment that the average person will ever make.  Most lending institutions will require the opinion of an expert to place a market value on a piece of property.  This is to insure that the buyer and lender are not paying too much for a particular home.  You wouldn't want to purchase a piece of property only to find out later that you couldn't sell the same property for as much as you paid for it.  Neither does the bank.

Other reasons to get an appraisal include :

• Refinancing - When interest rates drop below what you are presently paying on your home, you may find that you can save thousands, even tens of thousands of dollars, over the life of the loan if you refinance.
• PMI Removal - PMI or Private Mortgage Insurance is a policy that most lenders require if the amount of the loan is more than 80% of the value of the home.  This protects the lender, not the homeowner and could add $50 to $100+ to the monthly payment.  If you have owned your home for over 2 years, it may be that the equity that you have accumulated due to principle repayment and appreciation will cause your LTV (loan-to-value) ratio to drop below the 80% range.  An appraisal can prove to the lender that PMI is no longer necessary and save the homeowner hundreds of dollars per year by dropping the policy.
• Property Tax Challenges - If you feel that your property has been assessed too high, then you have the right to appeal the assessment.  Do not go to the appeals board unarmed.  An appraisal by a qualified, licensed appraiser will carry a lot of weight when appealing ad-valorum taxes.
• Estate Liquidation - If you have lost a loved one and they left a home or property to the heirs, an appraisal may be necessary to determine the "true value" of the property for disposition.  This way, all parties involved with any "potential" disputes have the piece-of-mind that an unbiased opinion of the property value can bring. 
• Pre-Sales - Are you interested in selling your home or property?  Unlike a realtor, a professional appraiser can determine the "market value" for your home.  Appraisers have no vested interest in what your home may sell for.  They offer an unbiased opinion of the price that your home should bring in todays market.