I don't know if I fully understand what you are asking about Business Units, but I'll try my best. If you choose different timezones for different Business Unitsm then you will have different timezones for different Business Units. Maybe I misunderstood you because this is just a basic fact. Regarding impact, there is no impact from having different timezones for different Business Units. Since Business Units are independent entities, they each will use the timezone that they have configured.
I don't quite understand what you mean by unique for Management Units. Yes Management Units are unique at the Org level, but I don't know how you could perceive them as not being unique, so I may be misunderstanding this question.
Regarding Business Units is it necessary to have different timezones instead of same timezone at all Business units. I am asking in which scenarios we will observe different timezones.
I saw some customers configure same timezones for all their Business Units. Some customers doesn't follow this ex : One BU configured in UTC, Other in EST.
Regarding Management Units
Consider BU1 and BU2 are Business Units.
I can create MU1 Management unit in both BU1 and BU2 instead of allowing me to create in either BU1 or BU2.
Why some customers choose the same timezone and others use different is completely up to them. No Genesys Developer can answer this. You'll have to ask customers yourself.
If you create a management unit with the same name in different business units they are considered different unique management units. They each have their own unique ID which is what should be used as the primary identifier, not the name.
I imagine that if a customer has their workforce spread out in different timezones, they will have multiple BUs with different timezones and only put their agents in Business Units that are their respective timezone. While some other customers may have all their workforce in a single timezone and so all of their Business Units use the same timezone.