The following should give you some idea why the existence of different clefs is a great thing:
I play both violin, viola and piano, so I am used to 3 clefs, G-clef, Alto clef (C-clef) and F-clef.
I am very happy about the alto clef for the viola, because if you used either G-clef or F-clef you would need a lot of leger lines, and if you wanted to avoid those leger lines you would need a lot of indications with the number 8 showing that the music is supposed to played in a different octave and that is annoying on a stringed instrument in the violin family, because a different octave means a totally different fingering on the instrument. It is much nicer to read the music in the right octave. That is also why a shift of clefs in a viola part from C-clef to G-clef if the music goes up on higher notes is much better compared with an 8va sign.
If you ever arrange music for strings avoid 8va signs at all costs. Exception is if the music goes above 5 leger lines which can happen in advanced violin music. With more than 5 leger lines in a violin part you need to count the leger lines which can be annoying. With 5 or less leger lines you can see right away how many lines there are without actually counting.
In cello parts use the tenor C-clef and if it goes very high the G-clef.
For the piano: Well, the pattern is the same in any octave, so an octave sign with a dashed line (or just a line) is some times needed and it is easy to play the notes in a different octave. With the line you can always see in which octave you are supposed to play. But do not use a clef with the number 8 or 15 above or below, because then you would constantly need to check whether the music is supposed to be played as written or should be in a different octave. Exception is if the whole part is supposed to be played in a different octave, but in that case you better write an explanation at the start of the score, because piano players might not think of the little number above or belowe the clef since they are not used to look for that.