By peeking at your competition, you check if there is anyone doing what you are going to do. If not, good. If yes, you can make your offer unique from scratch.
While researching keywords, you'll think what people using those queries were searching for and how likely they will be to complete the desired goal on your website.
The website design will depend on how the audience is succeptible to the visuals. If your target audience is women and you sell furniture or expensive clothes, you'll need a bright, fashionable website with plenty of high resolution photos. If you are targetting librarians, you might as well stick with text.
Obviously, the website will be structured around the site goal, along with the click path and information scent.
Depending on the goal, you'll create the content around it and your target audience.
Of course, making it extremely easy to complete the goal should be great. You can measure the time it takes to complete it, as well as the percentage of visitors that complete it (conversion rate).
By making your website accessible, you may also increase your customer base by 10-20% (the amount of people with disabilities in US/UK).
And, naturally, all the above, including the goal, will influence how successful you are with promoting your website, as well as the tactics you use to market it.
Just as well your site will be highly targeted and will be perceived as very useful to your audience, ultimately leading to increased conversions, whatever the conversion is.
Rounding up
Regardless of where you are in improving your website, having a clear goal in mind always helps.
If you don't have one (the goal, or both), investing time, efforts and money in the website will probably not pay off, unless you do it for entertainment, learning something new and other intangible personal reasons.