When it's done well, it gives you some guidelines, your wardrobe is more effective as you can mix and match the colours more easily.
When it's done badly, then it's not helpful.
Nobody should stick exactly to their palette - the palette you're given is a guideline and a tool only, not the be all and end all - it's about finding colours that work together and flatter you, rather than a fixed lot of 50 colours that are all you can wear.
We all stray outside of our ideal colours, but it makes for a much more effective wardrobe when all the colours have similar colour properties, so they can mix and match.
Plus the colours that suit you make you look more vibrant and alive, healthier and younger - so why wouldn't you want that?