It is possible to give a gate pulse on triac directly by using something that produces gate pulses. A DIAC is just such a device and is doing just what you suggest. A TRIAC gate is not designed to switch at a tightly controlled voltage or current and if a rising voltage source is connected to the gate the TRIAC will turn on in a "soft" and ill defined manner and time location. The DIAC allows input voltage to rise to a wellish defined level and then dumps a more than high enough voltage, energy source (capacitor) into the gate.
