What is the purpose of making Iskiate from Chia seeds?
I personally have not read the book, though I know someone who did and remember hearing of the drink.  Strike 2, I am also not a nutritionist.  Here's my best guess by looking at each piece:

Certainly the ingredients do bring taste to the combination, but I don't think it is just that.

Mixing the chia seeds into a drink may make it more agreeable for people.  Easier to stomach, so to say.  The chia seeds will absorb the liquid.  With this, you will feel fuller.  It will also make the drink thicker.

Lime juice is high in vitamin C.  This can help with muscle soreness.  Lemon can be used instead of lime - also high in vitamin C.  

I've heard the sugar is optional, though it will sweeten the drink and add calories.