Visual Composer gives you the ability to create attractive WP pages in a user-friendly interface. The interface focuses on the use of "blocks", which are effectively div's that can be styled automatically, without fidgeting with HTML, CSS, and JS.
Because of the super-friendly interface, it can be hard to make technical changes. If you need to make a change to the actual code, you have to go through a few different steps, or find a work-around for whatever is broken. This can be frustrating or confusing, so you need to be careful about any global changes you make to the website.
Also, some blocks or block contents are harder to implement than others, so you'll have to play around with the options to get a good feel for the process.
Though this will speed up your initial creation time, you may find it adding time down-stream. Whereas you used WordPress's "Edit this page" button to immediate make changes (such as broken links or copy edits), you'll now need to go into Visual Composer to make those changes, which will add a few more clicks to the process.
Visual Composer reduces dev time when building websites. As an agency, being able to produce functioning, attractive websites is our bread and butter. By reducing the amount of time it takes to create these websites, we can fit more projects into our schedule as well as reduce lead times for our clients. Visual Composer can speed up our web design process, allowing us to grow our revenue stream.