You will find the answer to this question in the PMBOK Guide itself - particularly in the introduction to the knowledge areas. For instance, this is from Time:
On some projects, especially those of smaller scope, defining activities, sequencing activities, estimating activity resources, estimating activity durations, and developing the schedule are so tightly linked that they are viewed as a single process that can be performed by a person over a relatively short period of time. These processes are presented here as distinct processes because the tools and techniques for each are different.
So the question isn't WHO does it but WHAT are they doing, HOW are they doing it and WHAT skills, T&T etc. are being used.
So is there a distinction in PMBOK that processes are divided based on who will be executing them? E.g. for a small project, a PM would combine Direct & Manage process and Monitor & Control process activities. Where as for a large project, a group of PMs would Direct& Manage their parts, and a Senior PM would take Work Perf Info and do Monitor & Control for overall project.
Take a look at Figure 4-7 on page 84 of your PMBOK Guide. Here you will see that the Project Document updates and the Work Performance Information are used as inputs to different processes. That is why they are called out separately.
As per PM PrepCast on Direct & Manage Project process, the listed Outputs of this process includes "Work Perf Info" and "Project Doc Updates". Arent "Work Perf Info" part of Project Docs? Why are they mentioned separatly?