It is a precursor in the umbelliferone biosynthesis pathway.

Umbelliferone is a phenylpropanoid and as such is synthesized from L-phenylalanine, which in turn is produced via the shikimate pathway.

Finally an intramolecular attack from the hydroxyl group of C2' to the carboxylic acid group closes the ring and forms the lactone umbelliferone.

The enzyme 4-hydroxycinnamate decarboxylase, induced in bacteria species such as Klebsiella oxytoca, works also with p-coumaric acid analogs such as E-2,4-dihydroxycinnamic acid.

[1] The dictionary definition of umbellic acid at Wiktionary