Let’s say you are an average but competent associate and you bill in the 50th percentile. But then you make it to your 6th year and you realize “whoa maybe if I lock in I can actually either (a) make partner at my current firm or (b) make a case to be a lateral partner at a lower firm”.
To be clear, this is in contrast to the associate who wanted to be partner as a junior and always strove to be top 10% in their class