Johnson's essay on biography in Rambler 60 comprises essentially a how-to on writing biography, which Johnson believed was one of the highest forms of literature:

and therefore no species of writing seems more worthy of cultivation than biography, since none can be more delightful or more useful, none can more certainly enchain the heart by irresistible interest, or more widely diffuse instruction to every diversity of condition.

In other words, a well-written biography could be not only entertaining but also a learning experience.  The belief that literature should instruct is a constant theme in Johnson's own writings--in Rasselas and Lives of the Poets, for example--and he believed that a good biographer had the ability to describe, through the example of the subject's life, either good or bad behavior.

Johnson argues in the essay that good biography should not restrict itself to people who play a large role in historical life.  Rather, Johnson makes the case for writing about the lives of men who have no public role in the belief that

We are all prompted by the same motives, all deceived by the same fallacies, all animated by hope, obstructed by danger, entangled by desire, and seduced by pleasure.

Because all men--great or not--are subject to universal feelings, emotions, and dangers, a biography of any man can provide useful object lessons on how, and how not, to behave.

In paragraph 7, Johnson pursues this argument further by noting that

but the business of the biographer is often to pass slightly over those performances and incidents, which produce vulgar greatness, to lead the thoughts into domestick privacies, and display the minute details of daily life, where exterior appendages are cast aside, and men excel each other only by prudence and by virtue.

The behavior and incidents that make men "great" are not, according to Johnson, necessarily appropriate subjects of biography.  Instead, Johnson argues that the biographer should focus on the "minute details" of a man's life when the biographer can discern how a man behaves when his defenses are down and his public persona is not operating.

Johnson argues that the biographer must avoid the publick picture of a man--that image gained by what the public knows of a person--and, instead, talk to a man's servant:

and so little regard the manners or behaviour of their heroes, that more knowledge may be gained of a man's real character, by a short conversation with one of his servants, than from a formal and studied narrative, begun with his pedigree, and ended with his funeral.

This instruction results from a deeply-felt belief, based on Johnson's own experience with the rich and powerful,  that a man's the true nature is displayed by how he functions in his private life and, more important, how he treats people--servants, for example--who are at the lower levels of the chain of being.

Ultimately, Johnson argues that good biography is not just a reflection of "regard to the memory of the dead" but to "knowledge, to virtue, and to truth."