You'll use it anytime you need to know the frequency of a signal to a resolution of up to 6 digits. Many digital scopes have a built-in hardware frequency counter of similar resolution. If yours doesn't, then that's what you'd use the 1900 for.
That model can also keep a running count of the number of pulses it sees. So, if you ever need to count something like trigger events from a device, especially when they occur very quickly, it's useful for that too.
If you don't think you'd ever need to know the frequency of a signal to that level of resolution again, nor have to count electrical pulses, then you don't need the device.