What function does the pde4 inhibitor provide for the body?

Lots of possible functions. PDE4 degrades Cyclic Adenosine MonoPhosphate (cAMP) back to plain old AMP. All cells in the body require cAMP, so inhibiting it inhibits the breakdown of cAMP. You can read more about it here: https://en.wikipedia.org/wiki/Phosphodiesterase-4_inhibitor and cAMP here: https://en.wikipedia.org/wiki/Cyclic_adenosine_monophosphate

PDE4 inhibitors are drugs that may help with a wide variety of conditions that cause inflammation in the body like psoriasis, chronic obstructive lung disease, and inflammatory bowel disease. Some examples of PDE4 inhibitors include roflumilast and apremilast.