Coffee is brewed once ground and it is important to know the right grind size for each type of brew. Whether you are drinking an espresso or a pour over, grind size is critical. Different brewing methods call for a different grind size as it affects the extraction speed and time. Finer the grind, more is the surface area which comes in contact with water, quicker will be the extraction. This is why espresso calls for a fine grind to make up for the short brewing time (25-30 seconds). A coarser grind in espresso, which is brewed under high pressure, will result in a much faster extraction and water will pass through coffee grounds quickly giving you under-extracted coffee. If you make the grind size too fine, you will get a slower and over-extracted espresso.
French Press, that takes 4 minutes to brew, requires a coarse grind to keep bitterness to a minimum and to limit the amount of coffee silts in a cup.