The PC-7 Mark II has an unpressurised cockpit. Meaning masks all the way. Flying at high altitude will be uncomfortable. A critical tactical minus in armed version. HTT-40 has pressurised cockpit.This will allow its pilots comfortably during weapon missions over Himalaya like terrain.
The mask provides two critical functions to pilots: it supplies oxygen to them, and it contains a microphone.
The microphone allows them to talk to the other person in the plane (if it's a dual-seater like a trainer), over the radio, and over the intercom (e.g., to ground personnel). You'd have to yell over the roar of the jet engine if you wanted to talk to your back-seater without the mask on. The mic audio is also recorded if it needs to be reviewed later.
The regulator supplies the pilot with breathable air (a mixture of oxygen and nitrogen, the ratio between the two depending on altitude) or 100% oxygen for emergency scenarios.
It can also provide a function called partial-pressure breathing for G (PPG), which pushes high-pressure air into your lungs during high-g maneuvers, which increases g tolerance. Why trainer aircrafts can do without mask is clear hear. They can not pull high-g maneuvre, since they have smaller power-plants, nor are they flied above 10000 ft.
In a cockpit above the altitude of 10,000 feet, it would take a little time before pilot starts to feel the effects of hypoxia, so pilots can certainly remove their masks for short periods for comfort.
But It's a good idea to have mask facilities even on trainer aircrafts in the event sudden depressurisation of cockpit due to structural damage.
I think HTT-40 in its final version will have oxygen cylinder and mask at least for one person.