Many web developers use the Jquery library because it makes writing code a lot easier, as the new simplified method can make it possible for you to create with only 1 or maybe 2 lines of codes, unlike before, wherein you could consume about quarter of page.
The DOM manipulation allows you basically to do anything you want with a web page such as take the content away, change it or add something that makes the website appealing, using a simple script. For most web developer this is important, as the task is a lot easier because of this.
Now, if you use MySQL queries take note that you can use it by sending it to the database, without even reloading the web page. Making it less hassle compare to other system use. When it comes to validation, you can still use it for your own form validation.
In fact, gone are the days of static website so you should consider the tool you will use for making a site.