What is a Qsys BFM?

I still have some confusion regarding the concept of BFMs. We have so very many different type of components in Qsys that all use Avalon-MM or Avalon-ST e.t.c How can a single Avalon-MM BFM provide stimulus to all these different type of components to verify them? How does the BFM know what stimulus to provide to a component? I mean, whenever we create a design and write a testbench, we all write the stimulus for those blocks. The stimulus is going to be different for different blocks and it is going to depend on the nature of the block itself.

So how can a BFM replace all that work, that also a single Avanlon-MM BFM for all Avalon-MM components? It does not make sense.