We basically use user controls when we have to use similar functionality on different locations of an app. Like we use master pages for consistent look and feel of app, similarly to avoid repeating the same functionality and UI all over the app, we use usercontrols. There might me much more usage too, but I know this one only...
For example, let's say your site has 4 levels of users and for each user there are different pages under different directories with different access mechanisms. Say you are requesting address info for all users, then creating address fields like Street, City, State, Zip, etc on each page. That would be a repetitive job. Instead you can create it as an ascx file (ext for user control) and in this control put the necessary UI and business code for add/update/delete/select the address role wise and then simply reference it all required page.
So, thought user controls, one can avoid code repetition for each role and UI creation for each role.
Ascx-files are called User Controls and are meant for reusability and also for making complex aspx-pages less complex (lift out some part of the page). They could also be beneficial for something called donut caching, that is when you would like to cache a certain part of a page.
If you have a block of code+html that appears on several pages and is sort of independent of that page (say a block of latest news items), you could copy/paste the code to every page.
It is however better to put that code in its own block and just include that block on every page that needs it. That "block" is an ascx file.
When you are building a basic asp.net website using webcontrols is a good idea when you want to be able to use your controls at more then one location in your website. Separating code from the layout ascx files will be holding the controls that are used to display the layout, the cs files that belong to the ascx files will be holding the code that fills those controls.
For some basic understanding of usercontrols you can try this website
One more use of .ascx files is, they can be used for Partial Page caching in ASP.NET pages. What we have to do is to create an ascx file and then move the controls or portion of the page we need to cache into that control. Then add the @OutputCache directive in the ascx control and it will be cached separately from the parent page. It is used when you don't want to cache the whole page but only a specific portion of the page.
ASCX files are server-side Web application framework designed for Web development to produce dynamic Web pages.They like DLL codes but you can use there's TAGS You can write them once and use them in any places in your ASP pages.If you have a file named "Controll.ascx" then its code will named "Controll.ascx.cs". You can embed it in a ASP page to use it:
Source: Stackoverflow.com