Website modernization is a restauration of the online appearance.
Imagine that your website is an antique musical instrument.
A fully functional, but a bit of a false instrument, which is in need of renovation.
And you do not want to change it dramatically, just a little bit more modern magic.
Why modernize your site?
Because you already have a working website established, which - in addition to development costs - already reversed much time and energy. Accustomed user interface of the content management system, recognized the limitations of the system.
You exactly know what you want otherwise, but is also aware there is too much time and effort would take the whole thing from the beginning to start.
Probably the website earlier developer proposes a new Web site design - regardless of whether the website is proprietary or free content management system (Wordpress, Jomla Wix, etc) - as the existing content management system development, updating, no longer, or significant energy input feasible.
Fortunately, today's modern content management systems all have their own "Template" interface, where you can easily using pre-made templates to change the site's appearance, layout, operations.
You do not want compromises because of limited options for content management system, the expected appearance and functionality.
You already have a logo, corporate image, accustomed homepage layout, you may insist on certain letters, colors and functions. Unfortunately, in most cases, your expectations and needs only a complex fashion or not at all possible to modify the template.
Natural expectation on your part that the website not only look good on mobile or tablet, but can also be used.
While mobile browsing sure you already annoyed many individual web sites of the size of buttons, menus, operation or as e-mail address or phone number you enter. During the development of the new website it is worth paying attention to them, in fact.
The location of the search engines acquired do not want to lose.
The hit list in the search engines (Google, Bind, etc.) are based on the content of the existing website, which regularly checked, evaluated and ranked.
That is why the current content, images, videos must be "without error" to transfer to the new content management system, which was time consuming, involves serious data loss risk, since website there are parts, pages and features that are not visible when the page is a normal appearance, however, the search engines robots ( like Google bots) have found a new web page and will also earn.
In total, it can say that if you want to improve the existing site functioniality while retaining the advantages of that, it is better modernize the existing website instead of a new web site.