One could argue that the subplot of Ben Jonson's Volpone lies in its existence as comic relief. Typically, comic relief, something used by many Elizabethan playwrights (like Shakespeare), is used to lighten the mood of a play (in one scene). Typically, the action has been revolving around sinister actions, deceit, and/or death, and comic relief allows the reader or viewer to take an emotional break from the seriousness of what is going on.
Prior to the introduction of the subplot, the play focuses upon Volpone's dishonest actions in amassing his fortunes. His latest con is convincing others that he is extremely ill in order to have extravagant gifts bestowed upon him.
The subplot happens in the following act—act 2. It is here that readers/viewers meet some of the secondary characters. For some, the second act illuminates the corruptible nature of society as a whole. Given that the vices of mankind are, essentially, split up between numerous different characters, their "flaws" do not seem as damning as Volpone's (since he possesses many). Instead, we are allowed to find humor in the behaviors of these characters because they seem to be more of a character flaw than anything.
We are not necessarily supposed to like the characters, but they seem to be far less intrusive and sinful as Volpone. Given this, many find that as the scene plays out, a little humor lightens the overall mood.