Why, for Plato, do forms necessarily exist?

Forms must exist as there are certain qualities that exist independent of the human experience. For example, my tie has both light blue and dark blue on it, so do my trousers. These items are different materials, different shades yet I still understand that they are blue, they are all instances of blueness, which I recognise. Since I have an understanding of the blueness that these items exhibit, a form must exist; as without a form tying these things together, I would not be able to know with certainty that these things all displayed blueness. Forms must also exist since humans have a conception of perfection, despite never having experienced perfection. Since humans can only imagine and conceive that they have experience are paradox is created: the solution to this lies in Plato's doctrine of from. For example, no-one has ever experienced a perfectly straight line or a perfect circle, yet we recognise circles as circles and straight lines as straight lines, despite them being far from the perfect ideal that we have in our head. The only explanation for Plato is that we recognise them because they relate back to the form of a perfect circle/perfect line, which grants us the idea of a perfect circle/line.