When I was new to portrait lighting, I used to blast everything with light with reckless abandon. Although, as I grew as a photographer, I learned that the ability to control the spread of light on a multi-strobe shoot allowed you to not only create something with a more sophisticated appearance, but tell a better story. One of the most affordable ways to accomplish this is with the use of grids. The Slanted Lens put together a great video tutorial on understanding grids. Read below to learn more.
The great thing about grids is that not only do they do an amazing job at controlling your lights when used properly, but they are cheap and easy to throw in your lighting case. Barely a photo shoot goes by when I do not use a grid for something, whether it be to splash a beam light on a background or create a gradient, to bright out details on a portrait subject, or to even create a narrow focused beam to pop the subject out of their environment. For these reasons, I highly suggest that if you own strobes, you should also invest in a few grids and play with them. Oh, and the grid for my beauty dish is one of my favorite modifiers to use when doing moody single-person portraits.
Below are a few examples in which I used a grid in a portrait scenario: