BAY 73-6691 is primarily used in research settings to study its effects on cognitive function and memory enhancement. It acts as a selective inhibitor of phosphodiesterase 9 (PDE9), an enzyme that regulates cyclic guanosine monophosphate (cGMP) levels in the brain. By inhibiting PDE9, it increases cGMP, which is associated with improved synaptic plasticity and memory formation. This makes it a valuable tool in neuroscience for investigating potential treatments for cognitive disorders such as Alzheimer's disease and other neurodegenerative conditions. Additionally, its role in modulating cGMP pathways has sparked interest in exploring its potential therapeutic applications for conditions like depression and schizophrenia.