Why does FLIR offer OEM camera modules with and without shutters?

For the majority of applications, a shuttered version of OEM camera is the best option. 

All uncooled microbolometer-based cameras drift with temperature changes, and for optimum image quality the pixels occasionally need to be re-normalized. This process is called a Flat Field Correction, or FFC. The parameters for FFC are time and temperature change.

The shuttered versions of FLIR's OEM cameras perform this operation automatically, however the user has some control over the frequency of FFC by way of setting the amount of time between shuttering, and/or the amount that the ambient temperature can change before shuttering. 

In certain situations where the scene or the system is always moving, a non-shuttered version can be used, as FLIR's OEM cameras employ sophisticated software that will compensate for this drift. However, an initial FFC should be performed using a uniform object or source in front of the lens (even a hand in front of the lens will work). 

Without details of the intended end-use of the camera or system, FLIR's guidance is to specify a shuttered version of OEM camera.