Client: We need feature X within the current site
You: Ok I will just make it a modal
When to use a modal
A modal should be used for critical blocking interactions.
The New York Times does this well.
Your modal has one thing going for it, it blocks everything else on the screen.
Do not abuse this fact – use it to your advantage.
You are fighting against the mass of websites that have trained users to instantly dismiss modals. They are used so widely to deliver social cruft and or advertisements that they have become a useless mechanic.
A key example of hitting the user in the face with a modal
Instead of modals think how to integrate said feature within your UX.
Do not rely on the quick fix that modals are – don’t be lazy.