Level I CFA: Economics: Monetary and Fiscal Policy-Lecture 3 

Its spend not save, i.e. spend=consume

To control the inflation. as we will increase the tax rates, aggregate demand comes down and which will pull down the GDP.

In stock markets after a point you start realizing that stocks are highly overvalued and a correction is expected (may be even wished for reluctantly so that it wont turn into a bubble at a later point). Similarly when things like income levels and output seems to be unrealistically high they shall lead to all sorts of problems. Hence it is better to contract rather than facing a Crisis situation.

prevent speculative environment (price higher, output constant)

Yes, but it's a much shorter lag than fiscal policy.