Cocktail muddlers are essentially used to muddle cocktail ingredients prior to shaking or serving. Most cocktails that include fresh fruit would normally require the fruit to be muddled. The length of the cocktail muddler is really important consideration. Most bartenders would use a muddler that is 21 - 30cm long.
If you are planning on making a few fruity cocktails, it would be a good idea to purchase a fine strainer with your cocktail muddler. This will ensure you don't serve a cocktail full of seeds.