Most inspections are performed during the purchase process of a home, after a purchase and sale agreement has been signed. Your purchase agreement should state that the purchase is "subject to an inspection satisfactory to the buyer". An inspection contingency is usually for 5 to 10 days; the longer the duration, the better...especially if laboratory test results or specialty inspections are required. Information from the inspection is then used by the prospective buyer to decide how, or if, to proceed with the purchase process.
You may want to make a tentative appointment for the inspection even before you have agreed with the seller to the terms of the (preliminary) offer. If possible, you should arrange to accompany the inspector during the inspection and discuss the inspector's findings afterwards.
Home inspections are also very useful before and during a major home improvement or remodeling projects. This type of an inspection is used to help determine the scope of work and the specifications for that work. An inspector can help you decide issues such as: "Do I really need a new roof?" "Will the electrical service require an upgrade when I remodel the kitchen?" And, an inspector can assist you in managing the construction contract, verifying the quality of the workmanship and assuring contract compliance.
The inspector's job is to provide an unbiased assessment, and to provide the inspection client with the information needed to make informed decisions. As such the inspector must be a neutral party and comply with a code of ethics such as that of lawyer.