What are the purposes of a fluorescent marker?

A fluorescent marker allows researchers to directly target an epitope of interest and analyze its biological and biochemical properties. Fluorescent markers are useful in numerous applications, including determining nucleic acid content, cell sorting, identifying and measuring distinct populations of cells and cell surface receptors, quantifying enzyme activity, immunophenotyping, apoptosis studies, and calcium flux experiments. 

Fluorescent markers offer many advantages including stability over extended periods of time, higher sensitivity even at low concentrations, and non-interference with the function of target molecules. Using different fluorescent markers in the same sample also allows researchers to simultaneously observe several molecules at the same time.