How does the use of inert gas in fluorination of saturated aliphatic hydrocarbons make it less violent?
Let's say the gas mixture has something like 10:1 molecules ratio of inert gas : fluorine. The alkanes in the gas would have 10 times more chance of colliding with an inert gas molecule as compared to a fluorine molecule. This reduces the rate at which alkanes and fluorine react together, making the reaction a lot safer (fluorination is highly exothermic to the point where it can be explosive). 

The inert gases are basically a necessity, without which it's impossible to do this reaction safely and responsibly.