what is the purpose of a parking brake when putting a vehicle in gear is enough to hold it on a hill?

Only an idiot does not Use the Park Brake on a hill

I think it is called an Emergency Brake, however if you put the car in park and its on a hill then, engage the Emergency/Parking brake before you let go of the foot brake there will be far less stress on the transmission...  The purpose of this feature is to help you stop if your foot brake goes out...

when your on a hill waiting at the lights, in a manual its helpful to take off with out rolling back on a steep hill if you not that familiar with the car.

When you are sitting on a hill and waiting so engine is running because it is -40 below outside so some heat is nice to have.  I ALWAYS USE THE "into gear" method(when engine is OFF). However when facing downhill, I slip it into Reverse gear. When facing uphill, then I slip it into 1st.  In case of low compression. Never had a problem. Never relied on the parking brake EVER.   Then if I WANT, I can ADD the parking brake to it. As on FWD cars the transmission locks the front wheels and parking brake locks the rear wheels-so 4 wheels locked- -just like stepping on brake pedal for brake fluid to lock all 4 wheels. 50 years driving...all manual transmission.

When to pull the parking brake does not make a difference. Usually it is the LAST action you do.

That is Unsafe the Engine can override the Gear the Hand Brake or Park Brake is designed and Fitted to prevent a vehicle from Moring on its own

It’s a good idea to use the parking brake and leave the car in gear ( or Park).

If the handbrake operates on the road brakes, if you apply it when hot, the brakes can contract and the car could roll away.

A manual transmission only holds the vehicle from moving because of the compression of the motor and the 1st gear ratio. The parking brake needs to be used to help keep the vehicle from moving. An older motor with less compression will roll.

The park position on a automatic transmission is merely a tiny pin or pawl that can easily break or slip due to wear and should NEVER be used to hold a car on a hill.

The park brake MUST be applied by law.

And a MANUAL transmission CAN hold on a hill by putting it in a high gear gear but it also needs to use the parking brake to be safe by law since it still is not safe.

That is what I thought until I parked my 5 spd Wrangler one day left it in 1st gear and it jumped out - rolled back and along the side of a mini-van some time hours later.

you would have to ask a mechanic