There are lots of reasons to use OOP for designing an application, though I am far from an expert.
When it comes to most personal projects or school projects, OOP style development is rarely necessary. There's usually not much repeating of code, the code isn't too complex and separated into many layers and modules. That's where OOP becomes useful.
However, in the workplace OOP becomes a bigger deal. When you've got dozens of project files, thousands of lines of code, and a complex workflow, OOP helps keep it all comprehensible. It allows for easier organization, less repetition of code, more readability, etc. Applications built on .NET MVC are a good example of this. Keeping things in separate layers of data (model), front-end (view), and function (controller) makes sure the code stays organized and that different modules don't overstep their boundaries.
For your last question, yes it can be a security issue. Part of application security is ensuring each module/segment of your code can only access the resources that it needs to access. Encapsulating your classes, keeping methods and variables private, etc. is a basic measure of security that can have catastrophic results if not implemented. This is actually the basis of a buffer overflow exploit, accessing resources that the module shouldn't be able to access.
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There are lots of reasons to use OOP for designing an application, though I am far from an expert.
When it comes to most personal projects or school projects, OOP style development is rarely necessary. There's usually not much repeating of code, the code isn't too complex and separated into many layers and modules. That's where OOP becomes useful.
However, in the workplace OOP becomes a bigger deal. When you've got dozens of project files, thousands of lines of code, and a complex workflow, OOP helps keep it all comprehensible. It allows for easier organization, less repetition of code, more readability, etc. Applications built on .NET MVC are a good example of this. Keeping things in separate layers of data (model), front-end (view), and function (controller) makes sure the code stays organized and that different modules don't overstep their boundaries.
For your last question, yes it can be a security issue. Part of application security is ensuring each module/segment of your code can only access the resources that it needs to access. Encapsulating your classes, keeping methods and variables private, etc. is a basic measure of security that can have catastrophic results if not implemented. This is actually the basis of a buffer overflow exploit, accessing resources that the module shouldn't be able to access.