Any pressure on the barrel can both minutely warp it, and change the harmonics. These are truly tiny changes, but at 100+ yards, that can translate into visible changes in POI.

A proper free-float prevents the forestock (handguard for AR's) from contacting the barrel at any point, and usually moves the sling swivel to the handguard, instead of its default position on the front sight base.

With no way for the sling or handguard to affect the barrel, the barrel tension and harmonics will now be consistent shot-to-shot (for similar shooting conditions and shot strings) no matter what stance you're shooting from, or whether you're sandbagged, using the sling, or just bracing against your body, etc.

Free-floating eliminates one of the many, many variables which can affect shot-to-shot POI, so it's definitely a good thing, but may not have much effect on your group size if you were already pretty consistent. Once zeroed, a consistent bias is no worse than an absence of bias.

You very well might notice a POI shift when you first switch over to a free-float handguard system, but once you've zeroed again, your stance or bracing should no longer have a measurable effect on your future POIs.