I'm thinking of buying some monitors if it will boost the sound quality but I already have like a 400$ speaker for the computer. Im wondering if it's worth or not for the investment
I'm not familiar with the logitech z906, but it seems to have good reviews, so it looks like it's a very capable full-range speaker system.
As you haven't identified a particular problem with them, I would suggest they are likely to be perfectly good for monitoring while playing your keyboard.
If you were to get a good pair of monitors, with a good amplifier, and site them and mount them properly in an acoustically-suitable environment, then you might find they had "better quality" than your Logitech in some way - such as a flatter response, for example. But that wouldn't necessarily make them any more enjoyable to play through.
A good keyboard amplifier will obviously be more portable and will probably let you play a lot louder than the Logitech system without distortion - but at home, you don't need anything portable, and there are presumably limits on how loud you can play anyway?
To quote Billy Joel,
Don't waste your money on a new set of speakers
You get more mileage from a cheap pair of sneakers
Next phase, new wave, dance craze, anyways
It's still rock and roll to me