The main advantages of this approach are its time-efficiency and the quality of the final result. For instance, if you're building a theme from scratch, you have to make sure to keep your theme up-to-date long after the development process has finished. The thing is, WordPress gets updated very frequently (around once every 50-90 days), and the purpose of many features changes over time—they might even get completely erased or replaced by new ones. In such a scenario, only modern themes that are kept up-to-date can take full advantage of these features. Updating your theme will obviously take a massive amount of work. This is where theme frameworks come into play. In essence, theme frameworks are themes with a very minimalist design and no visually-complicated aspects.
The purpose of frameworks is to make them the base of any future theme by acting as the parent theme. The fact that the framework itself has no design allows every creator to introduce almost any design imaginable, while taking full advantage of the features and constructions provided by the framework. Now, the strength of quality frameworks is that they get updated almost as frequently as WordPress plugins. In short, if you're using a framework, you don't have to worry about your theme going out of date. Whenever there's an update available, you can simply perform it and forget about the whole thing.
This brings me to the main disadvantage of using theme frameworks. Most of the time, theme frameworks are big, complex pieces of web software (Hypertext Preprocessor, or PHP scripts). So, if you want to be able to use them effectively, you'll have to spend a significant amount of time learning the framework itself. On top of this, if you decide to switch to a different framework later down the road, you'll have to learn all over again (frameworks are usually very different from each other). As you can see, reaping the benefits of using frameworks has its price.
Here are some of the popular theme frameworks (both paid and free ones), as follows:
- Gantry (free): http://gantry.org/
- Cherry Framework (free): http://www.cherryframework.com/
- Hybrid Core (free): https://themehybrid.com/hybrid-core
- Unyson Framework (free): http://unyson.io/
- Genesis (paid): https://my.studiopress.com/themes/genesis/
- Some frameworks are listed on the official WordPress website:
https://codex.wordpress.org/Theme_Frameworks