If cells are the units of structure and function in organisms then why are tissues and organs needed?
Eukaryotic Cells form tissues, tissues form organs, and organs form living organisms. All of these form different [related]! structures. The self-assembly of Biomolecules into sub-cellular components of cells - which then coalesce to form the Cells themselves - is performed through Biochemistry; Histology is the study of the development of Cells into Tissues and Organs; while Biology describes the 'self-assembly' of tissues and organs into Organisms. See Zoology.
Cell is the basic and most fundamental part of living organisms. Based on number of cells, organisms are divided viz. Multi-cellular(eg. all humans/animals) and uni-cellular organisms(eg. amoeba). Cell collectively form tissues, tissues form organs, organs form organ systems and organ systems collectively form a living organism.