The need is from PFIZER, Viagara patent is soon to expire. If, they acquire Avanafil, their marketing and sales machine could create a blockbuster revenue stream for another 10-12 years. Avanafil sets itself apart from Viagara, and thus offers a unique offering in this category of drugs. The generic Viagara will appear as soon as the patent falls. This is the motivation for them to make an offer.
One thing that allot people don't realize is these types of drugs have additional benefits besides thier obvious use in ED treatments. In addition there is allot of research taking place on other uses, See below