Also helps to prevent temp spikes in cars/trucks that have no bypass or the bypass is blocked.
Coolant is always flowing and the t-stat is less likely to open and close repeatedly until the engine is warmed up.
My 59 would warm up to 225 and then drop down to 150, then up to 225 and down to 160, up to 200 and down to 170----would keep on doing this until all of the coolant was up to operating temps (59 Elky with GMPP 454HO).
A few small holes in the t-stat and the engine now warms up slowly but evenly.
As Bob said, it helps to get air out of the system. GM systems are vented and must have the resovior in working condition so the vent system can work correctly. It says in every GM owners manual don`t fill it all the way up, fill it to the point it`s about 1/4 low, replace the cap, fill the resovior to the "full cold" mark and start the engine. After a while if you look at the resovior while it`s running you`ll see air bubbles coming up, then you`ll notice the coolant level go down. Replace the coolant as needed, keep it on the "Full hot" mark until it no longer needs anymore. It`s did this way to assure the cooling system venting is working correctly. I use a 180 degree thermostat and drill one 3/16th hole in the outer paremeter. Some will recommend 3 holes, but I`ve found if I drill 3 it flows too much and you have to run the engine hard just to get it up to temp.