I’m sure you run then, because again I have no idea what you mean by “splits”
In theory, you can pace yourself with a watch and a map. “Splits” are (distance, time) pairs that correspond to a targeted finish time. For example, if you were targeting a 4:00 finish at an even pace, you’d want to be crossing the halfway point at 2:00.
In practice, it’s mentally much easier to pace yourself off another person. The implicit peer pressure to stay with the pack helps when you might otherwise start to lag behind and helps you resist the temptation to blow up your plan because “you’re feeling good” and want to go for broke.
eg: https://www8.garmin.com/manuals/webhelp/forerunner245/EN-US/...
Holistically, pacers are just experienced runners that provide even splits over their distance. In a marathon context, running in a group can be quite pleasant and motivating, but there is definitely nothing stopping anyone from just using a GPS watch.