sf25
Re - descent - I think the reason is that it is very difficult to hand fly an aircraft at high altitude. The aircraft needs to be in denser air therefore a descent is required. The ATPL guys will correct me I'm sure.
Aircraft travel in airways. Drop below your assigned Flight Level and you run the risk of hitting someone coming the other way at a lower level or colliding with someone going the same way - again at a lower level. A 45 degree turn gets you out of harms way quickly.