Most of the time, when you are applying for a home loan or refinancing, an appraisal is ordered by the lender. In this case, the lender will select and contact an appraiser for you.
Other reasons you might need an appraisal:
Establishing a sale price before listing your home, or in a private-party transaction
Settling an estate
Divorce
Buyout of a partnership in a vacation home
Removing PMI (Private Mortgage Insurance)
Appealing your Okanogan County property tax assessment