With an eagerness to find out why Bali is popular for its beaches, I found a Bali guide from Hillmannwonders that depicts a few reasons for its popularity.
As it quotes”
Bali has many long, palm-fringed, white-sand beaches kissed by clear, warm tropical waters. There are beaches for every taste. Some are packed with sunbathers and lined with convenient shops, eateries, bars, and other tourist facilities. Others are populated with big-wave-seeking surfers. Still others are serenely off the beaten path and shared by few people.
Best known beaches
West Coast - They face the warm Indian Ocean waters and are admirably oriented for sunset watching.
Jimbaran Beach - Lovely location. Favorite of sunbathers seeking a long, quiet, scenic tropical beach. Never a blanket of people, even during peak season. Luxury resort hotels fronting the beach are discreetly semi-hidden from view.
Kuta Beach - Deservingly praised for its length and white sands, though the beach in front of Kuta’s main tourist facilities can become packed.
Legian Beach - Adjacent and similar to Kuta Beach, though not quite as busy and commercialized.
East Coast - These lie along the body of water between Bali and Lombok Island.
Nusa Dua Beach is populated with many international-style five-star resort hotels. It can occasionally become moderately crowded.
Sanur Beach - Once Bali’s most popular beach. Still pleasant, though waves in recent decades have eroded some sand.
Candi Dasa Beach - Half as busy and developed as Sanur and Nusa Dua’s beaches, but has in recent decades suffered even more sand erosion than Sanur Beach.
North Coast - Lovina is the most tourist-visited destination on Bali’s north coast. It is known for its “black sand” beaches (”gray” would be more accurate).
Positive - The Lovina area is less developed and commercialized than tourism centers like Kuta.
Negative - The beaches are hardly tropical paradises. They experience the sometimes hot and humid tropical Bali Sea climate. “Lovina” is a collective name for a string of seaside villages.
Source:http://www.hillmanwonders.com/bali/beaches_surfing_bali.htm