Free floating is just allowing the barrel to attach to the upper and then not putting any other pressure or parts on it (with the exception of the gas block and tube of course). If the handguard, rail, sling swivel or any other part attaches to the barrel, it provides opportunities for inconsistent pressure on the barrel that will effect accuracy.
The VLTOR VIS system is an incredibly strong upper/rail system that prevents anything from contacting the barrel. One of the best free float systems available for the AR-15 platform
Plain and simple, if you care about accuracy you have to free float. It costs more but it is an easy choice to get a basic rail that lets the barrel go free. Maybe you will never be an accurate enough shooter to notice the difference and maybe you will only ever shoot from 1 or 2 positions and the difference will be negligible. The M16 A2 has killed many a person without a free floating rail and any shooter can still be deadly with a plastic handguard that attaches to the barrel. But…
If you are going to attach/remove items from your rail, or use supported firing positions, you need to let the barrel free float.
KAC "Masterkey". Over 5lb of awesome... Clamped directly to the barrel. This is as far from free floating as any weapons system I have ever carried.
As an extreme example, I used to carry a Knights M4 and I loved it (as a counter-productive side note it did not have a free floating rail but still outshot my Bushmaster XM-15, with harris bipod in every way at every range – I ended up just leaving the XM-15 in the armory). However I had many attachments and I hated having to change them up as it meant a re-zero each time. One of the attachments was a the Masterkey.
Badass? Yes, very much so and it did open doors. But it was heavy enough that I didn’t want to have to carry it all the time – it was so heavy I could dump a full mag on auto into a 16inch circle at 25m standing, the weight just held the rifle steady with little to no help from me.
That amount of weight latched onto the barrel just past the gas port adjusted my zero a lot. At times, over 14 MOA on my elevation. And not consistently. Sometimes I needed to adjust windage by up to 2-3 MOA, sometimes just elevation and elevation could be more like 10 MOA. I had to leave it on and clean around it as often as I could, but there were times I had to take it off and on, which meant re-zero. It drove me nuts.
Other attachments like a surefire light might only adjust the zero by 1 MOA or less but then you have to consider shooting positions. Using a C-clamp grip on a barrier, or cranking on your sling in the junkyard prone can all effect your zero with a rail that attaches to the barrel.