At some stages you may feel the need to create pixel-perfect mockups. Especially when you are working with remote teams you don't have other way to specify how your product look like to developers that don't pay that much attention to details.

Besides from pixel perfect I've been missing a product that allow me to be "screen-transition perfect" since being sketching on omnigraffle / sketch and having to use another product to mockup transitions is a really painful process.