What is the difference between building a web app with a framework vs without a framework?

I am currently developing a web app locally with plain PHP, no framework. The application allows a user to create an account and then create events, invite people to the event, and keep track of who is and isn't coming. When someone gets invited, they get added to the guest list and they receive an email containing a link to a page with a form where they can RSVP. It's a pretty simple application that I have been working on for a bit and plan to continue working on, adding more and more features.

The thing is, despite it being a pretty simple application, it actually feels like a huge project. I have quite a few files, some with hundreds of lines of code already and I haven't even finished the project yet. Every little feature requires what seems like a fair bit of complexity, and that is even attempting to be as DRY as possible. I have not learned a PHP framework yet but I am wondering if using a framework for future projects will greatly improve development speed and reduce complexity, or if a framework is just for the purposes of allowing an application to scale, etc. I am still fairly new to PHP web development and I'm not exactly sure what the use of a framework is, but I am hoping that it will allow me to write things a little quicker because I just didn't realize how much work even a simple app would be.

Any pointers or guidance on development workflows or frameworks would be great, thanks.