A row counter is another tool that I keep in my knitting bag. I have found that row counters are especially helpful for any pattern where you need to pay attention to row number. I find that these patterns are often for colorwork or cables.
A row counter is not super necessary to have because I could also keep track of my rows using a pen and paper. Using a row counter however, is more convenient. There are different styles of row counters. Another style that I have seen is made so that it can be placed on your needles so you don’t have to keep picking it up to change the row. The style I have is magnetic so that it can be placed on a project board, I often just set in next to me or on my lap.