This structure has not been highly beneficial for the organism
Explanation:
Vestigial structures are cells, tissues, and/or organs that have no apparent function. Vestigial structures are retained during the course of the evolution, but often they are degenerate and/or atrophied (due to disuse). In general, these structures are homologous to anatomical structures that play a specific role in evolutionarily related species. Some examples of vestigial structures include, among others, the presence of the appendix in humans and wings in flightless birds.
The cell membrane functions as a selective barrier. We call these kind of membranes as differentially permeable or semi permeable membrane. It can decide what substances can pass in and out. Therefore it can surround the cytoplasm (which is the inside of a cell) to separate the cell's content from the surroundings.