Why is it important for each cell to be surrounded by a cell membrane that can prevent large molecules from leaving the cell
The cell membrane acts as a barrier between the cell interior and external environment. In the absence of such a barrier, the large molecules from inside the cell will exit the cell. As an analogy, one might think of what will happen to large organisms in our body if there was no external barrier (skin & skeleton, in our case). In such a scenario, all our vital organs such as the liver, heart, kidneys, brain, etc. will fall off and we will not be able to carry out vital functions and will die. Similarly, the lack of a cell membrane will cause all its vital organellea (such as mitochondria, nucleus, golgi apparatus, endoplasmic recticulum, etc.) to move out of the cell. This will cause loss of vital functionality and the cell will be unable to survive.
In fact, the membrane also prevents entry of most of the molecules into the cell and has selective permeability.
Hope this helps.