Some sub part of the equipments won't be maintained with stock. For these sub parts, business may want only Maintenance history, they may not require stock count. In this case, Sub Equipments will be created & will assigned Main Equipment. Here, you can mention exactly which Sub Part is installed to Main Equipment even though you have multiple same kind of Sub Parts.
Some parts are changeable during Main Equipment Maintenance & repair parts will be kept in store for further service and yet again it will be installed to Main Equipment. Here, if the business wants to know the stock count of Sub Parts, then those will be maintained in Material Master subsequently those will be assigned in Equipment BoM. Even you maintain in BoM, you can't define which exact part has been installed in Main Equipment. Equipment BoM is list of components by which Main Equipment has been assembled.
There is very thin difference between creating Sub Equipment and Installing in Main Equipment & creating Material master and defining in Equipment BoM.
Note: For Sub Parts, even Material Master can be created with Serial Number Profile & Sub Equipment can be generated. So both features can be availed by having the Material Stock as well as Maintenance History.