Seems most observers I follow(that have been pretty accurate on this sort of speculation) think this is a signaling move to the Cohen's, Manaforts, and Flynn's. A much more overt one than Joe Arpaio.
That not surprisingly, is probably stupidly risky and not likely going to be that effective. Since all those people are also facing state charges(or potential state charges) and there are so many leaks on this ship, and Trump's people don't even know the extent of it, so they have no idea where to plug, and dangling pardons is already something Mueller has signaled he will consider attempts to obstruct justice.