Offset on sensors just happens. Temperature, design, whatever; sensors often have some kind of offset. Eventually, it gets removed; usually during conversion and often in software.
Signal generators, OTOH, probably don't have offset unless it is deliberately added. In some cases, offset comes in handy. Consider sending a DC coupled sine wave to a single power rail op amp. One way or the other, the midpoint of the signal needs to be shifted up to midway between Vcc and Gnd of the op amp. So, a little DC offset is added to the sine wave.
Good old algebra: 'y = mx + b' still rules!