In situations that require confined space entry (for more details about what are
confined spaces, please see the brief tutorial
here) Gas Monitors are absolutely essential. Most dangers in confined spaces relate to either lack of breathing air (oxygen)or exposure to toxic or explosive gases and vapors. Lack of breathing air can cause asphyxiation, whereas exposure to toxic fumes can cause death or disablement, or injury. Explosive vapors and gases can of course explode, blowing people and equipment to smithereens.
But you will be surprised to know that despite there being several good, reliable and working personal gas monitors, how few are really used by workers. They are not to blame however, it is either lack of knowledge of the part of their supervisors, or a "cost cutting" mentality. This can result in needless injury, death and invitation to liabilities by the company if it can be proven that a worker got injured simply because he was not provided with a portable gas monitor.
Incidentally, here is an excellent training resource on confined safety procedures that is in an e-learning format. It means that with just one such course, you can train your workers in confined space safety procedures.
