Why are narrowing the range of masses, and achieving at least a 3-sigma result, so important? Well, scientists aren't entirely sure where to look for the Higgs.
As Aidan Randle-Conde said at Quantum Diaries, "Like anyone else, physicists have issues with confidence," by which he means "the extent to which they trust a trust a measurement.... Our data are statistically limited, so we can never be 100 percent certain in any of our measurements."
That's why limits are so important in particle physics. The more physicists can narrow the target mass range for the Higgs boson, the better their chances of finally detecting its tell-tale signature.
There are two primary scenarios: one that involves a high-mass Higgs boson (heavier than 130 GeV, or giga-electron volts, up to around 600 GeV), and one that predicts a low-mass Higgs (between 114 GeV and 129 GeV).
Earlier this year, scientists with Fermilab's DZero and CDF collaborations presented results that limited the possible range of masses to between 158 and 173 GeV with about 95 percent certainty. ATLAS has ruled out 155-190 GeV and 295-450 GeV; CMS, in turn, has excluded 149-206 GeV and 300-440 GeV. (Symmetry Breaking has a nice explanation of the different approaches employed by Fermilab and the LHC in their hunt for the Higgs.) This significantly narrows the range of masses in which the Higgs might be hiding.