When in a company, we may not know the book value or 
depreciated value of Fixed assets.  In this case, we 
assertain the value of the assets based on the market 
value. This is done to get a correct possition of the 
balance sheet. sometimes in case of takeover or acquisition 
of a company.

when book value of asset is extraordinay low in compare to 
market value of asset then revaluation is done in books to 
bring the asset in book on right value
if value of asset in books is high in compare to market 
value concept of imparement of asset will be levid
sometime revaluation of done is done at time of selling of 
business amalgmt. etc

When the company wants to expend the business, then the
company take a plan against every thing in his business, so
that some times the company replaced some assets for new
business, then the company followed method of inadequacy, so
the company take revaluation of old assets and sale its. and
another depreciation method(lapse of time) vied also the
company revaluation of assets.