Investors & landlords

Still confused.  When I purchased the furnaces I could have taken the entire cost as an expense in the year purchased OR I could depreciate it.. right?  As an expense it's deducted from my income in that year and done  where what you are saying is that by depreciating it I get to take a bit every year until fully depreciated BUT at some point I have to claim the depreciation when I sell?   Sorry I'm not understanding..  Appreciate the help!