Combined Block and Poured Concrete Foundation

I found a property that was built in 1985, with a poured concrete foundation, but a block wall in the area of the fireplace in the basement. There are no structural issues with the concrete foundation or the block portion of the foundation from what I can tell, but I am unsure why a builder would use a poured concrete foundation with a block wall. Any thoughts?
House is in Massachusetts.

Thanks!
Adam