When a Contact or Company has a multi-select property, during a merge the values of the source objects are not "copied" to the destination object.
For example, Company has a custom multi-select project named MarTech stack. Company 1 has values "Hubspot", "Google Analytics" and Company 2 has values "Mailchimp" and "Matomo". When these companies are merged, you want to maintain all 4 values.
With an import you can choose the bahavior by adding a ';' (to add) or without semicolumn (to replace).