Enrichment analysis approach (also called over-representation analysis (ORA)) was introduced to test whether pre-specified sets of proteins (e.g. those acting together in a given biological process), change in abundance more systematically than as expected by chance. This type of analysis investigates hypotheses that are more directly relevant to the biological function, and can also help highlight a process over-represented within a subset of proteins.
Further reading: “Huang DW, Sherman BT, and Lempicki RA (2009) Bioinformatics enrichment tools: Paths toward the comprehensive functional analysis of large gene lists. Nucleic Acids Res 37:1–13”