Many areas have rules regarding the use of open fires and fire pits. When wild camping for example in places such as Dartmoor National Park and the Lake District, it is very important that you do not have any sort of open fire on the ground as this leaves a scar that takes years to fully heal.
Using a firepit lifts the fire off the ground ensuring that no mark is left on the ground’s surface, it is however very important that you allow the embers to cool and extinguish fully before disposing of them in a way that ensures that no trace of your visit into the outdoors is left.
It is important that before you visit an area with the intention of using a portable fire pit that you do your research, look up any rules and follow them correctly.
You can find this sort of information in places such as national park websites and guidebooks.