Often, reactivity and reaction mechanisms are pictured as the behavior of isolated molecules in which the solvent is treated as a passive support.  However, the nature of the solvent can actually influence reaction rates and order of a chemical reaction.
Performing a reaction without solvent can affect reaction-rate for reactions with bimolecular mechanisms, for example, by maximizing the concentration of the reagents. Ball milling is one of several mechanochemical techniques where physical methods are used to control reactions rather than solvents are methods are methods for affecting reactions in the absence of solvent.