In some parts of the US, public transportation is sufficiently unreliable or inflexible that many employers prefer not to hire people who will be using it to commute. (This once-common question is now illegal to ask, but poorly-trained interviewers may continue ask it anyway.). (Source, in part, the California-specific https://haelaw.com/unlawful-drivers-license-discrimination/ , and in part my personal past experience of a time when "must have car" was common in job ads in other states.)
This question may also be a proxy for constraints like "are you available to work overtime on short notice", though of course there are many reasons someone might not be able to work extra hours.