Hullo Xin,
In case of a manufaturing company or a capital intentise company I think an onsite visit is very helpful to have a look at the facilities, check the type of assets they have, the culture and environment of the site and the people and their attitudes and informal language.
In the case of retialers, it si very important to visit the most relevant shops or slleing points they have. Even through mistery shoping. Talk to customers, employees, look at the organization of the shops, check the products, see what cusotmers buy…..
In case of financial companies is probably less relevat since you are buying a loan portfolio and financial assets and may have online services, but it is relevant to visit the most important branch offices and see the operations.
Regards
Pedro