Temples are similar to schools. There are temples at the neighborhood level, the district level, and the city level. Temples are schools of morality for the communities and there is a need for a large temple that acts like a university for morality, like a central for education for all other temples to help spread the benefits to the people all over the country.
A large temple is, thus, the central for innovation and event planning to spread Buddhism, to inspire excitement towards the development of morality. It is like the locomotive that pulls the train into motion.
Building a large temple that brings together people to study Dhamma and practice meditation is very important and beneficial as long as it is true to its purpose.
When all factors are considered, large temples such as the Dhammakaya Temple is more economical than the cost of building just one large university.