Is arranging in piano some what of basic ? or essential or fundamental for arranging in orchestra or any other genre with multi instruments?
Do you mean on piano, or for piano? Basic keyboard skills are required of all music majors at university (or at least it was a degree requirement in my day) so yes I would argue that being able to arrange a song for piano reduction is somewhat of a "basic or essential or fundamental" skill that all musicians should possess. Piano is very linear and visual, which is why it's a great first instrument for young beginners. Arranging on a keyboard (on piano) can help you to see melodic and harmonic intervals in a way otherwise not possible on (for example) a trumpet or clarinet.
Like what could you learn from just arranging your music in piano?
Again, do you mean on or for piano? My first paragraph may answer this. I'll add that once you reach the intermediate level of piano lessons, you learn different bass techniques, and chord inversions. These are necessary knowledge for any good composer/arranger.
How would arranging in piano increase my overall arrangement ability?
I believe it depends on you and on your understanding of how piano music works, and how piano players think. If you approach piano with a guitar mentality, it will sound like a guitarist's attempt to play keys. Much like when a pianist who doesn't understand guitar tries to play guitar on their keyboard with a VST/plugin.
The point I'm trying to make is that if you are using the keyboard to visualize your music in a new way, then yes it will definitely help you to see those intervals mapped out in front of you. However if you are writing for a piano player to perform your work, then you must first study other piano arrangements to try and grasp the perspective of the performer.