You should use galvanic isolation of input signals if
a) their voltage range exceeds the max input voltage of the DAQ system (you can use voltage dividers but these will not protect against excessive voltage spikes)
b) the reference ground of the input signals is at a level different from the ref ground of the DAQ system (the latter usually is the ref ground of the host PC which in most cases is connected to 'protection earth' of the mains outlet)
c) the ref ground are on the same level nominally but if there are long lines between the signal sources and the DAQ system which may cause level shift.
