Its important for two reasons, though only one have to do with comparisons to the cap:

1. The CBA specifies certain percentage of cap has to actually be spent in cash each year.

2. Cap numbers can disappear (they can also linger as dead money). Cash spent is paying for the team you have, not the contracts you constructed.

I think PFT leaves out a few personnel maneuvers that affect cash vs. cap beyond pro-rated bonuses.