Are you taking lessons? Scales are usually taught on the violin one at a time and built up to one note at a time, and most teachers leave C Major for about the fourth or fifth scale a student learns since it doesn't follow the default finger pattern that is taught to beginners. Therefore, if you are getting into theory and how many sharps and flats are in a scale but still don't really understand the structure and purpose of a scale, something is wrong with how you are learning to play the violin. You are trying to read at a third grade level when you still haven't learned how to sound out words.
"what is the use of scales anyway?"
There are many, but unless you get into music theory, they really only have three purposes for actually playing the violin: learning/teaching intonation, intervals, and later shifting; warmup exercises; and demonstrating skills associated with scales, especially for auditions.
"does it mean that if you were to play a piece with all natural note,you will have to use the c major scale that start with c?or other any pieces complement to their violin scale?"
Often, but breaking the rules, so to speak, is common. In fact, all of the great composers vary a bit from the indicated key. You don't use scales to play pieces. Good for you, using "piece" instead of "song," by the way. You use keys. Scales are a specific arrangement of intervals. They are associated with keys that share the same name, and there are pieces that contain scales within them, but if you say a piece is in C Major you would be referring to the key, not the scale. A piece in C Major will have an empty key signature and will usually end with a C on the melody line. For the most part, there will be all naturals in the piece, but you might see a note here or there marked with a sharp or flat symbol. We call those accidentals.
"does different scales have different octave or something?"
They start on the lowest pitch that shares the name of the scale on your instrument. There are some scales you might learn in a higher octave first. For example, I teach A major as the very first scale my violin students learn, and they learn it in the second octave. Later, they learn it in the first octave and combine the two for a two-octave scale. Eventually, you will play three-octave scales on your violin. Once you are playing multi-octave scales, you will start in the lowest possible octave your violin can play in that key.
"whats the difference between any scales?the starting point?the fingering?"
Both. The intervals - the pattern of half steps and whole steps - is the same across all major scales. If you change it, you have a different kind of scale. All scales will be named for the starting pitch.
"and can you play any piece with just the c major scale?"
As I said earlier, you don't use scales to play pieces. You use keys. Any piece can actually be transposed to a different key, so as long as it was written in a major key to begin with, it can be played in C Major. There are also plenty of simple tunes that are pretty much built around all or part of a major scale. "Joy the World" and "Twinkle, Twinkle, Little Star" are examples.
"why dont piano have scales?"
Pianos do have scales. The only instruments that do not have scales are simple instruments that do not have pitch at all or do not span a full octave.