When done without cells, DE uses "in vitro" transcription translation to produce proteins or RNA free in solution or inside artificial microdroplets. This has the benefit of allowing more conditions (e.g. temperature, solvents). It can express proteins that would be toxic to cells. Furthermore, "in vitro" evolution experiments can generate far larger libraries (up to 10) because the library DNA need not be inserted into cells. That often limits what can be done. 