Increasing the surface area of a substance involved in a chemical reaction exposes more of the material to the reactant, thereby speeding up the reaction process. This is why chemists often use powder, filings and other forms that maximize surface area.
For example, one chemical reaction that occurs in nature is the reaction of iron with oxygen, which results in rusting. A solid chunk of iron will rust slower than a pile of iron filings. Similarly, hydrogen peroxide naturally decomposes into liquid water and gaseous oxygen. That decomposition process can be accelerated by using manganese as a catalyst. Powdered manganese results in much faster decomposition than a lump of manganese does.
The rule is that increased surface area results in faster chemical reactions, but there can be exceptions. For this, an example is a typical reaction of a solid with a gas. Suppose a chemist wants to burn magnesium. Gaseous oxygen is a necessary factor of any combustion reaction, but if the magnesium is in the form of a very fine powder, the oxygen molecules will have difficulty penetrating to the interior of the magnesium pile, as the magnesium oxide product forms a crust over the magnesium and chokes it. The chemist achieves a faster reaction in this scenario if the magnesium is a long, thin strip.