Wookbench Requests are those in which you add client open contents to it, whereas Customizing Requests are those in which you can add client specific data.
Client Open means the change will be done in all the clients available in SCC4, whereas client specific will be only for that specific client.
Imagine a scenario where you have 3 clients in your development server (800, 810 and 820 which can be seen in SCC4). Where I have set 800 with Automatic Recording of changes in SCC4 for 800 client.
Imagine you do all the development activities in the development client 800 and you are requested to create a Plant. Now when I login to client 800 and create a new Plant, the system will ask for a Customizing Request. Once the Plant is created and saved in this request you will have to move the data to clients 810 and 820 in order to use the Plant in those clients, else it will not be available there.
Whereas in the case of Workbench Requests, when you complete a certain activity and save there, the system will have the data available in all the 3 clients. There will be no necessity to move the data to each client in this case.
But it is not that you add all contents to WB request or to customizing requests. There are restrictions for each and every single object. For example in MM all config except the access sequences captured done in Customizing Requests. Access Sequences are captured in WB Requests. Also all customized programs that you do create and move to other systems will require WB requests.
/Manoj