Put simply, ionisation is required when grounding is not possible, this includes:

• Process Essential Insulators (materials that cannot be grounded by nature and cannot be replaced with an ESD safe alternative)
• Isolated Conductors (A conductor that comes into contacts with ESD Sensitive Devices that cannot be grounded or equipotentially bonded together. Per IEC TR 61340-5-2, 5.3.3.3 Isolated conductors, "Examples of possible isolated conductors are; test probes, bond wires, hand tools and transport rollers).
• In area where high voltages are used (as an alternative to personal grounding... Learn More)

One other use for ionisation is to suppress electrostatic attraction (ESA). Point-of-use compressed gas ionisers neutralise charges that otherwise cause contamination or visual defects by blowing ionised air so that the contaminant particles are not instantly attracted back to the surface.

Air Ionisers work by creating a large number of positively and negatively charged ions and dispersing them via their air source. The compressed air or fan is then directed at the surface that requires neutralising. The ions from the ioniser then attract to the opposite polarity ions on the surface. As a result, the static electricity that had built up on the surface is neutralised. Ionisers are required to be balance in order to neutralise efficiently and not build up a charge on the surface instead.

Ionisation can neutralise static charges on an insulator or charged conductor in a matter of seconds, thereby reducing their potential to cause ESD damage.

Per IEC61340-5-1, INTRODUCTION, "Necessary insulators in the environment cannot lose their electrostatic charge by attachment to ground. Ionisation systems provide neutralization of charges on these necessary insulators (circuit board materials and some device packages are examples of necessary insulators)."