Lisp syntax is just that: syntax. It has nothing to do with semantics. In other words, the syntax (ideally) shouldn't affect how you write programs; if you feel like you have to "complete"completely forget how you write regular programs", it sounds like you're having difficulty with the semantics, not with the syntax.
Practical benefits of Lisp syntax include:
consistency. Function/macro/special-form followed by its arguments. No operator precedence issues. Grouping is usually explicit.
easy to parse and unparse. This makes it easier to write source code tools, such as static analyzers and code browsers.
extensibility
- user-defined macros. Users can extend their Lisp system's syntax.
- system-defined syntax. New special forms can be added to a Lisp system without having to worry about how their syntax interferes with existing forms.
You also threw in a couple of questions about side effects and functional programming. Those aren't related to Lisp syntax and should really be asked in a separate question.