Why does a cell need to maintain its shape? What happens if we take away the cytoskeleton from an animal cell or what happens if we take the cell wall from the plant cell?
Plants, specifically, would wilt, and all cells would suffer a decrease in surface area - to - volume ratio.
The plant cell is far easier to answer. Plant cells, at least in the stem, rely on turgidity to remain straight. The central vacuole exerts pressure on the cell wall, keeping it a solid rectangular prism. This results in a straight stem. The opposite of turgidity is flaccidity, or in other terms, wilting. Without the cell wall, the plant would wilt. Note that this is only taking into account the effects on the shape of the cell.
In an animal cell, the effect would be less visible if, again, we only consider the effects that the change in shape would have. (No cytoskeleton or cell wall would spell disaster for cell division!) The biggest problem would be a decreased surface area - to - volume ratio. A high surface area - to - volume ratio allows more things like nutrients, excretions, and secretions to enter or exit the cell. This is because more surface area exists across which molecules may diffuse, relative to the cell's volume. To have a high surface area - to - volume ratio, cells must be flat in shape and often dimpled or covered in villi. Without the cytoskeleton, the cell would naturally become spherical. To lose its shape would be to dramatically lower a cell's efficiency.