Planning is the most important part of any project, with planning you can scope functional requirements. You can plan on where to place certain elements of a website. With proper planning you are building the foundation of your website.

I've found that if you only show one page design to a client, for example the homepage, they often don't know what they want til later in the cycle. Beat this by wireframing each page that you will do, even if a client doesn't know what they want. Wireframe pages you think they will need. This helps with speccing out projects and can help avoid confusion at a later stage.

If planning is done properly, all parties will know exactly what to do and the project can be completed more smoothly.

Projects fail a lot due to poor planning. What happens is that planning isn't done, the website build starts and the client decides later that they want to change something. This becomes a vicious cycle of change after change after change till eventually the client can't afford to pay for changes anymore and you're left with a half completed project.

Make sure to keep the client involved from the beginning of the project, they are the main stakeholder and they alone know what they want. Organize planning and spec sessions. Get sticky notes and write down stories for the user (I'll cover a bit more on this at a later stage). Just make sure to get everything that needs to be done written down, this way you'll be less likely to miss out on something.

Plan plan plan.