My top-down vest pattern, knit on a large circular needle, called for markers to remind me when to increase... (Where the sleeves and the front and back sections meet.) When knitting a sweater bottom up in four pieces - front, back and two sleeves - the edges are physically present to remind me that an increase - or decrease - may be needed.
There are a lot of markers on the market. Over the years, I've used plastic ones as well as large safety pins. My complaints are that the round plastic markers tend to fall out, and the safety pins sometimes get caught in the yarn.