People with 'webbing' between their fingers and toes sometimes lack the same or a similar abscisic acid enzyme. In humans this is sometimes caused by a genetic defect, rarely by environmental factors (although; epigenetics, this is a debatable thing).
All multicellular organisms grow taller and larger from a single clump of cells (embryo or later in the case of plants one or more meristems). These cells need to be cleaved enzymatically to grow into organs and limbs. ABA is one of those enzymes, but there are a couple hundred more at least. If said organism lacks a certain cleaving enzyme, or the organism doesn't produce it for whatever reason, the organs develop as a whole, uncleaved. In most plants this is often temporary and usually localized (functioning is not optimal and plants tend to drop less functional parts off to benefit high functioning parts, aside from stopping infection through necrosis - ironically by cleaving the cells enzymatically off and shutting their sap flow down).
In humans, this mutation/defect can be pretty lethal because the body isn't able to remove malignant growths, so even though these people look very normal in most senses, they require regular checkups to make sure there isn't something growing that the body should've gotten rid of.
Yeasts, bacteria and more primal singular cells need these enzymes to divide themselves after budding, sporulating or copying themselves. There are some theories that the basis of multicellular life was laid out by a mutation that stopped these single-celled bugs from dividing into two, and growing into a larger multicellular organism.