The appraiser may represent either the landowner or the condemning authority. Usually, the government entity that needs the land for public use orders an appraisal and offers to purchase the land for the value indicated by the appraisal.
If the landowner feels that the amount offered by the condemning authority is not enough, the landowner may also order an appraisal. Landowners often do not consider this option, though, they usually try to settle with the authority by negotiation rather than incur the expense of another appraisal.
It is obvious that the landowner's negotiating position would be enhanced if the landowner did order a professional appraisal report from an appraiser of their choice. This appraisal report could be made available then to support the landowner's interest with the property's estimated value in an unbiased and professional manner.
If the parties still cannot agree on a price, then the matter will be settled in court with each appraiser testifying on behalf of their respective value estimates. Appraisers are not advocates for their client; they are expert witnesses trying to support their value estimates.