A part without any Schmitt trigger input circuitry will switch at the same voltage level for both rising and falling edges. This is problematic because switching draws current from the voltage source causing it to drop. This in turn changes the switching voltage level, which can make the part switch again. Oscillations such as these can result in substantially more current draw. Likewise, a noisy input could cross the part's switching voltage level several times, also causing oscillation and other problems downstream.
Placing a Schmitt trigger such as the Texas Instruments SN74AHC1G14DBVR between the input signal and the part solves these problems. The hysteretic window specified by the voltage thresholds the Schmitt trigger operates at negates oscillations. Also, by changing the slow input transitions into fast transitions, the rise and fall time requirements of the following circuitry are met. Further, a Schmitt trigger has no limitations on how slow the rise and fall times of the inputs can be.
Alternatively, there are parts with Schmitt trigger action such as the NXP Semiconductor 74HC1G14GW,125. Parts of this type have similar advantages but without all of the Schmitt trigger circuitry. They do have some hysteresis between rising and falling voltage thresholds, but there is still a specified limit to the rise and fall times of the inputs. While they do help reject noise and handle slower input transitions, there are limitations when not using a true Schmitt trigger.
