Keyboard amplifiers are required by musicians who perform onstage for an audience. The built-in speakers on keyboards are usually adequate for personal use (directly or using headphones) but not for an audience, may be for a small audience in your room but definitely not if you're performing in a pub or a bigger venue.
That is why you need a keyboard amplifier to pump up the volume. A keyboard combo amp contains both the amp and speaker in one unit and these are quite popular for performing. Behringer, Roland, Peavey, are the more popular brands that manufacture amps for keyboards.
Some of the models can be quite heavy so that is one factor you need to bear in mind, besides the wattage part, which is dependent on how big the venue is going to be.
For bigger shows, most keyboard players use their amps for self-monitoring and the actual output is send on the common PA.
So if you're planning to upgrade to a pro keyboard, make sure you buy a decent Keyboard amplifier.
