Grids can be very useful, but there are downsides. They're excellent when the light is behind the subject facing towards the lens, for reducing flare, though it's often just as easy to to block it off with a piece of card or screen or something. Grids are also handy for controlling spill - keeping the light where you want it and not allowing it to spread to where you don't, though TBH I'd say that's not often a major problem either way. Sometimes of course.
The downsides, that rarely get mentioned around these parts, is that grids eat light (and if you're using a speedlite, that's precious stuff) and they make the light
harder. So when you've gone to the trouble of getting a decent size softbox for nice soft light, you then go and throw half of it away! And the closer the subject is to the softbox, the harder it becomes (relatively speaking) as the grid cuts off an increasing amount of the usable softbox area towards the edges and corners.
Use a grid when you need one, but not when you don't