there are 2 reasons actually.. and both of them have already been mentioned.
1) in schedule 1 of spouse, it is claimed that the spouse has been working. so, it needs to be proven to ensure there is no misrepresentation.
2) a working spouse does not earn points, but it ill give a notional edge of adaptability to the IO to approve borderline cases.
As far as letters of reference are concerned, I don't think they are necessary from spouse's perspective. employment contract and salary proof should be good enough...