There are a variety of reasons. Someone may need to install to satisfy a court-ordered requirement—such as for probation or deferred prosecutions programs—or they may need an IID to retain their driving privileges as part of an administrative licensing requirement.
Other reasons to install an IID may include child custody and family visitation issues or work-related compliance requirements when alcohol use is a concern. Others may voluntarily install an IID to ensure children or other family members are not drinking and driving.