To understand the importance of B and C market in today's Tamil cinema first we need to know what these markets are?
So, A, B and C are used to tell the viewers in advance what they should expect from the movie. A means more realistic focus on logic and story, B means little less logic added with masala and focus on hero and C is all about masala and only focuses on the hero and present everything larger than life.
An article published in Hindustan Times gave a great example of how the same premise can be shown in A centre movie and in C centre movie.
The article's example stated, "You might say that there isn’t a single moment in Srimanthudu as powerful, as intimate as the one in Swades where the NRI protagonist buys non-bottled water from a little boy at a nondescript railway station. But then, that was a different kind of movie, the equivalent of a close-up — Srimanthudu, on the other hand, is a wide-angle shot. Swades devoted a lot of its running time to detail how this man changes this village, while in Srimanthudu, it all happens over the course of one song. But this isn’t about which is the better film. This is about how the same premise is treated in two modes of popular Indian filmmaking — a relatively ‘realistic’ mode ( Swades) and an unapologetically larger-than-life mode ( Srimanthudu). So Swades has a protagonist, while Srimanthudu has a hero — one who has to perform periodic goon-bashing duties in order to remind the mass audience that despite his do-gooding impulses he can be a badass."
Indian film audience especially the Tamil film audience visit the theatre to see something larger than life. The main aim of the audience is still to seek pleasure from the experience, they use cinema as a medium to escape their daily life and have something extraordinary in their ordinary lifestyle. They can appreciate one of two art films of realistic films but what they are addictive of is mass films and because of that more of mass movies are made in Tamil film industry in comparison to realistic or relatable film.
Hence, B and C market in today's Tamil cinema is important in both in terms of business aspect and in order to provide the demand of the audience.