Usability is very important in achieving goals over a platform, but it is compounded by different factors:
Catchy design and UI
Easy to handle and learn
A website or application that has been built with keeping in mind user’s ease will minimize the need for call and assistance. These designs are mostly free of bugs and errors. Treating problems in the design phase smoothes work progress in the future. Smooth experience also increases conversion, thus resulting in more traffic and transactions. With an increase in usability there comes a steep decrease in manual dependence. (Importance)
Face value is very important. Colour schemes and design skeleton should not be annoying to a visitor. It should be tolerable, if not compelling. (Usability in Different Platforms)
Secondly, the buttons should not be too stylistic or hidden so that a general user gets confused. It should be absolutely straightforward. Also, the buttons should function equally well on both desktop and mobile interfaces.
When the site works in the same manner, its homogeneity makes it memorable for the visitor.
And finally, a visitor must feel satisfied with the overall experience.
Satisfaction drives another chance of visiting the same site over and over again. So, when you have hired a developer, make it clear that usability is your priority.