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one of the selling points of amps with built in FX is that you are good to go. BUT in a live situation switching on the fly could be a concern which requires a spare hand or foot pedal controller. If you are just playing at home or jamming with friends there is not a lot to be gained. If however you are gigging you might find foot switched selections and specific pedals or brands give you a certain something, often you the player are the only one that can hear the difference in the mix but there you are. Also having all in the one box saves clutter and space.
Of course G.A.S. can grab you at anytime and cause you to buy more toys or effects as we call them.
A stomp box is a thing of beauty in itself, and the fact that it is also useful only adds to the Joy of FX. I always think that any unit with a wide range of built-in FX, be it a floor unit or an amp with built in FX, is brilliant for a player to mess around with effects, see which ones they really can't live without, and then take that beautiful leap of faith into the void and start buying individual stomp boxes. To my mind a lot of built in digital FX will give you a sort of generic example of any given effect, but without much in the way of character. For example, my own favourite chorus pedal is the Electroharmonix Small Clone, but I've never found a chorus in any digital FX unit which comes close, and the same with analogue delays (a distinctly non-digital effect, of course), distortion units - the list goes on and on ......
So, to summarise my rambling reply, I'd say you don't need to buy any other pedals if you don't want to, but a) I think individual stomp boxes will give you more character and so a more interesting sound palette and b) you DO want to!