Maintenance
Digital keyboards do not require periodic tuning as opposed to acoustic pianos which need to be given or at least should be tuned twice a year. They are easily cleaned and kept away for storage.
Portability
Digital keyboards are also much more portable to move around, as you might have already guessed. You cannot move or carry your acoustic piano with you whenever the fancy strikes you.
Extra features
A digital keyboard can allow you to play different kind of keyboard samples such as grand piano, honky-tonk piano, harpsichord, organ and the electric piano. Apart from this you can also experiment with dozens of other instruments such as guitar, electric guitar, flutes, strings etc. Even for a piano player, this can be entertaining and fun.
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Affordability
A digital keyboard is likely to be much more affordable than an acoustic piano. Even if you choose one of the high-end models that sell for a pretty high price, you are likely to get off by paying less.
Privacy during practice and playing
One of the great advantages of the digital keyboard and especially when you are learning it is that you can plug in a set of headphones into the keyboard and practice to your hearts content without the fear of disturbing the rest of the household or your neighbors. This means that you can practice that any time that is convenient to you even if it is early morning or late at night. This can be an immense boom for a busy person who is just starting out with his piano lessons.
Learning help & features
Digital keyboards also come with helpful guides and features that can actually help you learn the piano. For example, many keyboards come with a rhythm and beat maker which allows you to set the proper beat and play according to it. This helps you learn to play piano with proper rhythm which is an very important part of learning how to play.
Conventionally, many piano teachers ask you to count the rhythm by constantly saying tick-tock. A good alternative is using the beat ticker in the digital keyboard.
Many digital keyboards also have a chord finder. On the LCD screen available with many high end digital keyboards, you can see the chord that you are playing right there on the screen.
As I have already mentioned, the high-end digital keyboard come with touch sensitive keys. Touch sensitive keys produce louder or softer sound depending upon the intensity of your touch just like a regular acoustic piano. The 2nd feature is weighted keys. The keys on acoustic panel are made from material other than plastic and have their own mass and weight. This feel can be replicated in a digital keyboard as well. So the 2nd feature that you might want to look for is weighted keys. These 2 features would bring your closest to the feeling of playing an acoustic piano. While they may still not match the real thing in the quality of the sound, some of the latest models of digital pianos to provide a very decent and close alternative.
For many people, digital keyboard is the chosen option as opposed to buying a synthesizer. People are really not concerned with programming synthesizers, other than a few professionals. They are more than happy with the sampled sounds that are pre-stored. As long as your keyboard has the ability to produce realistic samples of other instruments, you should be pretty happy.