This is very common at least in MRO CS companies, Plant Maintenance it is not of importance as in Plant Maintenance you are not selling services whereas in customer Service you are any wherever you have Customer and Sales, Sales Order is important 
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b) Service Contract , Repair Quotation and Repair Sales Order contains: Pricing of Repair , Schedule of repair , Items which are part of contract , any Contract specific data and your repair price .In some case customer might ask for a Repair quote, the way we ask when we go to check for Car repair. If Quotation is approved then only proceed with actual sale of service. Also you can link your outbound delivery is linked to Repair Sales Order which contains the detail of shipping.
b) Service Order : If you pricing (Condition tab on Sales Order) is good and customer is all right with your quote then only create the Service Order.Service Order is more about the planned cost and Actual cost it does not have the option for discount etc.
To answer you question mainly Sales Order is used for Pricing Though you have number of other options.If you want to track or have multiple condition types like discount , preferred pricing for customers, warranty status related pricing etc then you go with Sales Order as Sales Order has condition type which you can use to arrive at Price of Repair .
Anjan