The complaint I have about my large speedlights like the SB-910 and SB-800 is that they are just too big and clunky. But they do the job and are excellent flashes. The alternative used to be use the little on-camera pop-up flash. The D500 and D850 don't have an on camera flash, so that option is unavailable, and I don't want to carry a big speedlight unless I plan to use it. The SB-300 fills the gap.
The SB-300 is smaller than the Nikon battery charger or a glasses case. It's small enough to keep in my camera pack all the time so it's always available. I can use it with an SC-29 flash cord for off camera use. It has a guide number of 59, so it is not that powerful, but is enough for a gentle fill flash. This is great for close up and macro work as well as casual portraits.
This is a simple flash - controls are minimal and generally it is controlled with the camera. The SB-300 sells for just $147. If you have an on-camera flash, it still can be useful to move the flash off camera or off axis.