Formula fields in Crystal Reports really empower you to manipulate your data in a variety of ways when a simple select statement can't do the trick.  The syntax is quite simple to use if you already know C# or VB and the help documentation is packed with samples.  Crystal also has numerous built in functions that you can use in your formulas.  There are built in functions for standard deviation, variance, Nth Largest, Nth Smallest, amortization, fair value, trim, and uppercase just to name a few.  You can also use looping structures such as for, while, and do loops.  You can even find the previous or next value of a field.  The power of the formulas and scripting language is tremendous and can allow you to accomplish almost any requirements defined by even your toughest users.