I will explain the below statement..
LOOP AT p0008ASSIGNING <fs_0008>. "For each loop <fs_0008>(like a pointer) will point to each record in p0008
<fs_0008>-BET01 = change required. "changing BET01 of current pointing record. So modify statement not required.
After this statement internal table p0008 will get modified.
endloop.
Try this in debug mode. See the changes.