benifit of wood chips or chunks ?

Most charcoal lump has some wood left in it, as well as broken down wood components that didn't get driven out of the charcoal mass. So a little of the wood smoke flavor will come out of the burning lump.

However, adding unburnt hardwood (chips or chunks) allows the heat to not only breakdown the cellulose (the bad white smoke) but the lignin that made the wood hard. The heated lignin breaks down into a number of components that are also found in some spices. As I recall, both clove and vanilla components are emitted, as well as others.

Most of the histories of BBQ that i have read say that the origins were fires built by Taino and Arawak natives in the Caribbean. The used allspice/pimenton wood. I have used some of that, and the aroma is almost incense-like. I didn't really like the flavor, but various woods actually do spice the food as they are heated.

Smoke can also provide additional preservation qualities when curing meat. The surface of the meat becomes a little more inhospitable to bacteria, drier (helps prohibit mold) and perhaps a tick less attractive to insects

We add it now because we like the flavor, because with modern refrigeration and supermarkets stocking meat year-round, we don't truly need to preserve and smoke meat in quantity.