Board thicknesses are measured in the "quarter system" - 4/4 (pronounced "4 quarter", that is 4 quarters of an inch) is a 1" thick rough-rough sawn board; 8/4 (8 quarter) is a 2" thick rough board. As this is rough sawn (super rough and unsightly finish), you usually lose at least 1/8" while finishing the board, so a 4/4 rough board gives you something around 3/4" finished product.
This leads to 5/4 being another fairly common dimension, as you can get a 1" finished board out of it. 6/4 and 12/4 are somewhat common as well.
It might seem like an "ugly dimension" compared to something like m^3, but its really built around speed. Watching someone skilled price out an enormous stack of mismatched lumber (width, thickness, and length) in seconds with a lumber rule is always a fascinating process to watch (https://www.popularwoodworking.com/techniques/the-lumber-rul...).
Usually the $/board foot increases slightly at higher dimensions as thicker lumber is harder to produce. I got some 4/4 black walnut for $12.80/bdft last week; 8/4 was closer to $16.
Edit: This is also why 2x4s are actually 1.5"x3.5" - historically they were 2x4 rough sawn lumber, but .5" is lost in both dimension when producing finished boards you purchase in the store.