Spiders eat various vermin and protect crops, and therefore Filipino farmers depend on them for pest control. As spider-fighting has gotten more popular, the Filipino pest population has steadily grown. The dangers aren't entirely limited to crops, either. Being a tropical climate, the Philippines boasts an impressive number of mosquito-borne illnesses, and spiders help control the mosquito population, as well. Spider fighting is not limited to the Philippines.
Japan has a long tradition of spider-fighting, dating back hundreds of years.
Every year, hundreds of enthusiastic spider-trainers convene to pit their trained arachnids against one another in organized matches, eventually winning the honor of battling the town mayor's champion spider. Although there are probably plenty or people who think of it as cruel, spider-fighting and other similar sports are a part of Asian culture and identity, and part of what makes Asian culture unique.