Well cell membranes are made of Phospholipid Bilayers and they are only selectively permeable. The Hydrophilic ends face the extracellular space while the hydrophobic ends face away from the extracellular space and arrange themselves with other hydrophobic tails.
This prevents any water soluble, Hydrophilic substance from entering the cell without a channel protein. However, Hydrophobic molecules can pass through the membrane easily.
This allows osmoregulation. If the cell membrane wasn’t semipermeable, water from the extracellular space would rush into the cell down it’s concemtrarion gradient and cause it to burst.