Honestly don't know why anyone cares about these figures unless they have shares in the company
They are important because the camera companies think they are important.
When Sony purchased the rights to Minolta's A-Mount, they saw it as a way into the DSLR market - and were reasonably aggressive with the price and functionality of their initial DSLR models (A100, A200, A300, A350).
They made some headway into the market, and soon had 15-20% market share - BUT, when they failed to get above this, they decided they needed a change of direction, and went for the DSLT models (but still with A-Mount).
This failed to make the progress they wanted, and they switched again to E-Mount - then FF E-Mount, which finally got they sort of sales figures they wanted, and we see the benefits with the rapid growth in terms of new models and lenses.
Without the sales figures this would not have happened, and if Sony find their market share slipping back at any point, they will look for the next way to turn things around.
Sales figures are important, as they are what drives the decisions on the future of the products.