In brief:
Stock Requirement List: It is a dynamic list, which shows the current status of requirements & sources for that material. So any changes in the procurements or needs will be shown in this list.
MRP List: It is a static list, which shows the status of requirements & sources just after the MRP run. This list is not updated till the next MRP run.
Now the qns, why 2 lists:
The 2 lists helps the material planner to compare the changes which has taken place post MRP run. So comparing the 2 lists the planner can then make an informed decision whether the MRP run scheduling has to be done more frequently or whether it can be less frequent.
For more details, you can read the SAP help files.
Hope the above was of help.
Regards,
Vivek