long final
1-why dont i just as for a RIS everytime.
It's a case of responsibility on your part. If the weather is superb CAVOK yet everyone asked for a RIS, with the amount of aircraft that get airborne on superb CAVOK days then the service would be impractical for the Controller to give - you'd just swamp him. Bear in mind that if there is IFR traffic on a RAS he HAS to give that his utmost attention. Your requesting only a FIS can help him to achieve that. If you are not happy with the inflight conditions then by all means ask for a RIS - hazy conditions and flying into a low winter sun are the prime conditions that spring to mind that a PPL without an IMC rating can still find himself in.
2-when does it become obligatory to be provided with a RIS rather than FIS
It is never obligatory - it is up to YOU to decide the level of service YOU require.
CM