If it's a project you never expect anyone else to pick up then that's probably fine... but frameworks make extending, maintaining and updating projects better by keeping code organized and concerns separated, assuming multiple developers will be working with it over a long period of time.
Like mixing jquery (or any js) into PHP... perfectly valid and probably close to what it was actually meant to do, but with a framework (on top of PHP) you might have an asset loader to manage scripts and dependencies or put the script tags into view templates to keep the markup separate from the logic. In a few years if someone has to come in and maintain your code, they might be familiar with it in the context of a framework, versus having to parse everything mixed in together. Something like changing the ORM or changing the template handler becomes much easier because the framework provides that extra level of abstraction.
They also attempt to create organized and universal solutions for general use-cases, so that with minimal effort using the same framework and tools you could make any number of CRUD apps. For a lot of people it's worth the complexity to not have to deal with the overhead of picking and installing libraries or writing an ORM of their own, probably very similar to the "convention over configuration" idiom in RoR. Sometimes the price you pay for this is configurability and speed.
Like mixing jquery (or any js) into PHP... perfectly valid and probably close to what it was actually meant to do, but with a framework (on top of PHP) you might have an asset loader to manage scripts and dependencies or put the script tags into view templates to keep the markup separate from the logic. In a few years if someone has to come in and maintain your code, they might be familiar with it in the context of a framework, versus having to parse everything mixed in together. Something like changing the ORM or changing the template handler becomes much easier because the framework provides that extra level of abstraction.
They also attempt to create organized and universal solutions for general use-cases, so that with minimal effort using the same framework and tools you could make any number of CRUD apps. For a lot of people it's worth the complexity to not have to deal with the overhead of picking and installing libraries or writing an ORM of their own, probably very similar to the "convention over configuration" idiom in RoR. Sometimes the price you pay for this is configurability and speed.