No efficient disinfection without a good cleaning:

Improper cleaning can overwhelm high-level disinfection regardless of subsequent steps.
Cleaning is the most critical step and may result in a 4 log reduction in microbial contamination or bioburden.

Since March 2001, the French regulation requires 2 washing steps and successive rinses before the endoscope disinfection, after the first mandatory manual pretreatment besides the patient's bed.

A good cleaning is the only way to limit the biofilm development that may be very fast between the patient examination and the endoscope disinfection.

A biofilm may form on any surface exposed to bacteria in an aqueous environment. Only 10% of the natural bacterial population is growing as free cells in suspension ("planktonic cells") in an aqueous medium. Most of bacterial cells are "sessile", i.e. attached to a surface and living within a biofilm. Once bacteria adhere to a surface, cells are able to deeply change their behaviour, building true cells communities well protected behind an excreted slimy coating (the "glycocalyx"). Reducing their exchanges with the outside, these particular cells all together behave as a significantly different organism, and are quite out of reach of disinfectants or antibiotics.
As they are always viable in their planktonic form, sessile cells represent a wide potential source of recontamination when re-dispersed in the environment and are implicated in a significant amount of human infections.