What did the president know about Iraqi WMD—or, more to the point, what did he think (or what was he led to think) his intelligence agencies knew?
This is why the Senate Intelligence Committee wants the summary released. It's the same reason the 9/11 commission wanted the White House to release the president's daily intelligence briefing of Aug. 6, 2001 (the one headlined, "Bin Laden Determined To Strike in U.S."). They want to know what the president knew. Did he have reason to see Osama Bin Laden's attack coming—and, if so, should he have done something about it? Did he know about internal disputes over the evidence of Iraqi weapons programs—and, if so, should he have thought twice about going to war?
If all George W. Bush knew about the Iraqi threat was gleaned from a one-page summary that stated the case for WMD—and that did not even acknowledge the existence of a case for skepticism—that's important to know. It's important for citizens who want some insight on why we went to war. And it's important for the president, who may decide to read a longer document the next time there's trouble.
Emphasis added by me. *ahem*. Especially when that longer document is written at his request, expressly for him.
A National Intelligence Estimate is not an ordinary report. It marks the one occasion when the Central Intelligence Agency warrants its name, acting as a central entity that pulls together the assessments of all the myriad intelligence departments, noting where they agree and where they differ. Most NIEs are produced on an annual basis. Occasionally, the CIA is asked to produce what used to be called a "special" NIE. The 2002 estimate in question, titled "Iraq's Continuing Program for Weapons of Mass Destruction," was such a document. It was ordered so that the president could decide, in an informed manner, whether to go to war. The president is the main consumer of the NIE; it is written entirely for his benefit. To shrink the thing into a single page—to remove all distinctions between certainty and guesswork—is to evade the whole point.
*ahem* Emphasis, again, added by me.