in general you maintain a purchasing group in material master and from there it will flow to the requisition. if there is no material master, then the requisitioner has to enter the purchasing group manually.
The rational behind that is, that the requisition gets assigned to the purchasing group which is responsible for the procurement of this material/service .
the buyers convert requisitions to RFQs or purchase orders, e.g. in ME57. And they can use the purchasing group to find all requisition that are created with that purchasing group.
if a requisition is converted to a PO then the purchasing group will even flow to the PO,
if the buyer is not identical with this purchasing group, but has the authority to work with requisitions of an other purchasing group, then it makes sense that this buyer enters his own purchasing group instead of keeping the original assigned, because he will be the contact who knows the business case in case the vendor has a question.
it is nowhere predefined whether the purchasing group is a group of buyers or is just a single buyer, this is usually based on your own design, and can be used to segregate duties and authorization.
Just imagine you have mixed design, you use real groups, e.g. raw material, trading goods, MRO items for internal use in requisitions, but use dedicated purchasing groups per buyer for the external usage in RFQs and POs.