Under what circumstances is it better for a cell to have a rigid shell, and under what circumstances an elastic one?

In primitive animals, the soft shell facilitates the movement and capture of food particles (amoeba – pseudopods), the growth of primitive organisms; a person has leukocytes in the blood, which perform a protective function similar to amoeba, they perform a protective function – they surround and capture foreign bodies and destroy them.
Hard shells are necessary for creatures with a constant body shape, if these organisms have organs for locomotion. Rigid shells also play an important role in protecting cells from damage and squeezing.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.