In addition to what's been stated, what you can gain by client side checking is better usability. Web forms are a good example. Using something like JavaScript, checking can be done very quickly on the client side. This saves on the number of calls to the server application by stopping the client from running the server application with incorrect data and allows the user to see their mistakes right away without having to wait for a process to run - which would display an error about the incorrect data - then having to go back and do it again.
But like you point out, someone could modify the client (turn off JavaScript, etc.). So server-side checking remains very necessary as well.
Rich36

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There's more than one way to screw it up...

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