Why are high pressures used in cracking of long-chain hydrocarbons? If we have a long-chain hydrocarbon, such as decane, and we split it through thermal cracking (say in an industrial plant), we use high temperatures, and high pressures. Cracking produces smaller molecules - alkanes, and alkenes. It might look like this: 

$$\ce{CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3}$$

Which is cracked into products like this:

$$\ce{CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3-&gt; 2CH2=CH2 + C3H6 + C8H18}$$

Why do we use high pressure in this case? Le Chatelier's principle states that the equilibrium will shift to resist the change — in this case it would move the position of equilibrium to the left, not the right. In cracking we want maximum amount of products (ie. the right) — therefore surely a low pressure would shift the equilibrium that way.

The reaction is not dependent on molecules colliding — merely enough thermal energy has to be provided to enough molecules to cause them to split — homolytic fission — and become free radicals which react to form smaller molecules. Therefore a pressure requirement to increase the collisions doesn't seem necessary.

Why then do we use high pressure when cracking hydrocarbons?
First of all, cracking is not an equilibrium process, therefore La Chatelier-Brown principle doesn't apply! (irrelevant)
Also, the mechanism is much more complicated than this:


In gas phase, radical reactions generally have extremely complex reaction networks. E.g. burning of methane with oxygen contains thousands if not more elementary reactions. I am pretty sure that gas phase cracking is not a pure first order reaction.
In industry, cracking is generally done on heterogeneous catalyst, therefore not a gas phase reaction. Heterogeneous catalysis is pretty much controlled by the absorbed amount of molecules on the catalyst surface: high pressure, faster reactions.