It can hold exceptional importance in certain situations, especially in the case where a targets products are produced by an external manufacturing partner.
The established contracts and IP ownership may be readily available in the data room but the actual conditions of a facility, and their true processes, can be critical. The cost and time to take a flight could not only help uncover optimization opportunities that could be implemented post deal but avoid a surprise in quality, cost, or reputational risk. Even in the case where internationally recognized auditor data may be available a visit by the buyer, who knows the product / industry process exhaustively, can see things others following a checklist may miss.