The "purpose" is to fill an NSA datacenter in Utah through voluntary, liability-free sharing, and to make defense contractors rich. Nobody wants to let the NSA siphon their data because they got sued last time. This bill is a liability shield, so even companies not really interested in sharing all that much are dumb to turn it down.
Edited to add: The NSA, in cooperation with the DHS, has been running the pilot program related to this for several years. AT&T, the largest non-defense contractor throwing money at this, is helping them. There are billions in government contracts and quasi-regulatory-capture (note the language about certified cyber security providers) waiting to change hands over this. Then everybody can share with the NSA, customers lose all rights, and don't even know its happening. This is a legalized version of the warrantless wiretapping scandal.
The Democratic (note before you yell: I am one of those) alternative to CISPA is the one designed to make defense contractors rich. It created a stratum of professional service firms more or less deputized by the USG to conduct mandated audits on private-sector systems determined by some process within the USG to constitute "critical infrastructure"; it was, in other words, a giveaway to Raytheon and SAIC.
Actually, it's the opposite. The original CISPA was the free-market deregulating alternative to Rockefeller. It's taken on more bipartisan support over the last year (but that's now receding, I think due to the climate in DC).
You're right that it's not a dichotomy; I'll try not to portray it as one (as much --- the failure of CISPA does set the Democrats up for another bite at the apple on their proposed regs, which are OMG worse).
Edited to add: The NSA, in cooperation with the DHS, has been running the pilot program related to this for several years. AT&T, the largest non-defense contractor throwing money at this, is helping them. There are billions in government contracts and quasi-regulatory-capture (note the language about certified cyber security providers) waiting to change hands over this. Then everybody can share with the NSA, customers lose all rights, and don't even know its happening. This is a legalized version of the warrantless wiretapping scandal.