This question is too vague to answer properly. If you are using server side processing to execute scripts on the server or access database information, it is essential. If you are using it strictly for the include feature, it depends on the size of your website.
SSI saves you time when updating common elements on each page of your website, but the creation and maintenance of the included files can be difficult and time consuming. Since the included files are usually incomplete code snippets, many editors and browsers will not handle them properly. This can restrict you to using something like notepad to maintain your site. If you want to view your results you will either have to upload them to a server first or run server software on your local PC.
If your site is only a few pages, the extra hassle is not worth it, if it is huge, it is worth it. The crossover point is up to you.
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Aside from the access of databases, server-side scripting languages (preferably PHP) you can make a webpage extremely dynamic. PHP and ASP have include functions that allow you to accomplish the same idea as old-school HTML frames in a more consistent, standard, sustainable way.
You can add username/password features, as well.
Secure forms.
The main advantages are user interactions can be managed on the client and reduces load on server.
In a large web-based application, where you provide both a web browser user interface and low-level communication interfaces, then both strategies would be used.
It's good to have client-side processing, because that means that the user gets fast feedback, and doesn't have to wait for a server round-trip.
# The user doesn't need to install anything (read: not even PHP), beside her browser. # Security