This function smoothes out an incoming signal so that the change in voltage level cannot exceed a certain number of volts per second. This causes the result to “lag behind” changes in the input. It is sometimes called a slew limiter or technically as an integrator.
One common use of a lag generator is to introduce portamento (a glide between notes) when you feed a pitch control voltage through it. For this application, you’re fine with having one control for both positive and negative slew (a rising or falling voltage).
Another common use is to smooth out the sudden on/off voltage of a gate signal into a more gradual envelope signal. In this application, you would a prefer a lag generator with separate rising and falling rates – for example, to convert a gate signal into an envelope with fast attack (rising voltage) and slower decay (falling voltage).