Power
Any time you make decisions involving money, you have a certain amount of power. As an appraiser, you're deciding what someone's home or potential home is worth. This directly impacts things like how much down payment is required, what kind of loan the client qualifies for, and how much profit the seller makes.
Let's say a couple buys a small home to live in for a few years until they decide to have children. The house needs some updating and the couple plans to go all out making their little home the showplace of the neighborhood. Fortunately, when they bought the house, they decided to seek advice from the person who appraised its value.
As someone who understands the marketplace, you're in a position to tell them how much they can expect to make off the updated house when they sell it in five or six years. If they spend too much and make the house much nicer than other homes in the area, it's likely they'll lose money on the update.
In smaller towns, there may only be one or two appraisers working the area, meaning you are likely to know your clients before they become clients. This is where things get tricky. Someone may want you to over-value or under-value a home for a friend, depending on their needs, but you can't ever do that.
The valuation you place on someone's property impacts the surrounding properties. Using your power to unfairly help one person could hurt many others. Not to mention that if a questionable valuation goes to court, you could lose your license over an unfair appraisal.
Power should always be tempered with ethics and sound judgment. Otherwise, you're no better than the flavor-of-the-month pop star who acts like a jerk because he believes throngs of screaming fans will continue to follow him into old age. You know, old age—like twenty-eight or so.