You can analyze the philosopher and their work. Socrates' system of thought can be understood without knowing he fought in the Peloponnesian War, that he was beloved, and that his wife was a shrew. But his philosophy makes even more sense when you know those personal details because context matters. For instance David Hume was an empiricist but accounts of his life make little to no sense. Besides motivations, personal details usually illustrate the limitations to a philosophy. That being said I don't think of Foucault as a philosopher, I think of him as a rhetorician and charlatan.