The truth is that your customers have limited time, and perhaps even more limited attention. We use Calls To Action (or CTAs) to help potential customers find the most direct way of doing business with you. That might include making a purchase through an e-commerce platform, signing up for a service, or contacting you directly to discuss their needs.
The more intuitive the process, the less your potential customers need to think about the ‘how do I?’ and ‘when should I?’, and the less likely they are to hesitate or put your offer on the back-burner, lost amongst dozens of never-to-be-revisited bookmarks.
To demonstrate, I visited some of the most popular websites to see what their primary CTAs were and how easy they are to find:

Twitter leaves us in no doubt as to what we need to do. It gives us a clear, concise intro on the left, and our eye naturally takes us to the sign-up form on the right with the prompt ‘Join Twitter today’ and good contrast on the ‘Get Started’ button.
Gmail

Gmail entices us in with an image that shows how its services may be integral in our life, and a red ‘Create an Account’ button points us towards the form used to create an account.
Netflix

Netflix uses imagery to suggest what the current availability of shows are, many of which are easily recognisable by their title screens. They elegantly blend this in with their own branding, enticing us into creating an account for their services with a very bright red button, which works particularly well as the background is rather busy.
Paypal

Paypal’s front page has an attractive split-screen, catering for both the personal market and businesses at the same time. They’ve chosen a simple white button with blue text, but a colour overlay has been added to the photographs behind it so that the text and button are legible and noticeable.
Twitch

Twitch is an online streaming service aimed primarily at gamers. They welcome new members with a clean blue banner across the top which promises various bonus content when you sign up. In the purple bar at the top, there’s also the same ‘crown’ icon used with a notification, which when clicked shows you some of their enticing goodies. This one is by far more subtle, but since they offer a free service, they’ve dotted it all around the site in non-invasive ways to keep reminding you of what else you could be enjoying if you signed up for Twitch Prime (which just so happens to be included in Amazon Prime, too).
Change.org

Change.org wants to invoke an immediate action for an important cause, and using red on white pages sure brings that action to our attention! The page is bright and straightforward, letting the petition titles and images speak for themselves, while still encouraging visitors to participate and keeping it clear how they can do so.
CTA’s remove the guesswork for your visitors. It can also encourage them to take more immediate, definitive action. None of the above CTAs suggests the companies are desperate or sleazy because they offer value, a solution to a problem, or a way to connect and get involved.
Using CTA’s in your project
Depending on the rest of the branding, they can be subtle and gentle, or bold so that they’re noticed over a busy background. They don’t need to be loud or obnoxious (RIP flashing buttons of the 90s… and good riddance), but they should contrast well and draw the eye so that the visitor sees the clear signposting. Good design is about not making visitors think too hard!
So next time you’re browsing the web, make a conscious effort to take notice of various Calls to Action, how they fit in with the rest of the branding, and how they’re directing you on your journey. They’re more common than you think!