A free-float forearm is a great advantage when you are attaching a sling or a bipod, or are using a supported position (barricade or port) for resting the rifle. Without a free-float, any of the above induce some stresses onto the barrel which can deflect point-of-impact (POI); with a free-float, the stresses are on the forearm and receiver, leaving the barrel alone to do its work unmolested.
As an example, my g/f and I both shot a 3-Gun match this weekend. There were two 100-yard rifle stages. Both of us are zeroed at 50yd and have an offset of about 1.5in high at 100yd. One stage was shot on a table (we rested on the magazine, so were effectively shooting unsupported at the barrel), the other using a wall as a support for the rifle. I have a free-float tube and saw the same POI (expected +1.5in) on all targets. My g/f shoots a Kel-Tec SU-16C that has a non-floating forearm. She shot the expected +1.5in from the table, but from the supported position had to aim at the very bottom of the target to get hits---it was about a 12in high target. I'd say she went from +1.5in to almost +8in at 100yd because of the barrel deflection.