Lobster Johnson (and possibly Edward Grey) are thematic analogues to the B.P.R.D.
Mike Mignola has said before that if there had been a B.P.R.D. at the time, Lobster Johnson most certainly would have been a part of it. It is not a stretch, then, to say that Lobster Johnson was the B.P.R.D. of his time (albeit in a more One-Man Army way). The implication is this: defending against paranormal threats is a required duty, whether in a cosmic/fate sense, or for more practical reasons.
Lobster Johnson's thematic value then note is to provide an analogue to the B.P.R.D. And just as the B.P.R.D. seemingly has a higher calling sometimes in the performance of their duties, so too does Lobster Johnson have a higher calling in death to the greater goal of Justice (not to get in to Epileptic Trees, but this may explain his near-corporeal ghost form).
Thus, by including this character, we can see that paranormal activity, and defense against it, have always been necessary to the well-being of humanity. And we can also see that the need for it has escalated, when comparing the size of the B.P.R.D. as an organization to the Ragtag Bunch of Misfits that the Lobster employed.
Edward Grey (of the new Witchfinder sub-series) serves a similar thematic purpose.