The main lubrication on a modern chain is internal and for life. If the O-rings are intact, the grease can't get out and crap can't get in. However, the chain rollers engage and release from the valleys between the sprocket teeth in the course of the chain running on the sprockets. And this is the important part, chain tension. A chain that is too tight will be stressed and stretch prematurely, possibly even break. A chain that is too loose rides up the "hills" on the sprocket teeth in the course of use, constantly moving up and down, up and down. That is where the wear happens. Ideally, the chain tension is just enough to minimize this climbing or "working" action, but not too tight to prematurely stretch it. The lubrication is there to take some of the shock and wear out of the chain rollers contacting and then breaking contact with the sprockets millions of time each time the bike is ridden. Properly adjusted and cared for, chains last a long, long time.
RT