In a nutshell, a chronograph lets you measure the speed that the bullet leaves the barrel. This information, in conjunction with data about the bullet you are firing and the environment you are shooting in, lets you start mapping out the expected flight path of the projectile.
I ended up with around 100 rounds of 7mm08 and .308 to put down range over the weekend. Got to be happy with that.
I reloaded another set of rounds headed out to the range to shoot them off then came back and collated the information in OnTarget TDS.
Proper cheek weld is critical in long gun shooting accuracy. Why? Repeatability.