Giuliani was allegedly tasked with coordinating and executing the alleged impeachable conduct on behalf of the President.
Lev Parnas and Igor Fruman allegedly were directly involved with Giuliani's Ukraine activities, which means they may be valuable fact witnesses. At the very least their testimony relates to the firing of Marie Yovanovitch, which has previously been considered relevant to the impeachment hearings.
Additionally, they may have information that allegedly implicates high Ranking Republicans in the President's alleged impeachable conduct.
Them being charged provides leverage to convince them to testify, which may further support exisiting witness testimony and evidence in removing the repeated use of the "alleged" label in this answer, as well as lead to the discovery of even more witnesses and evidence.