Taking an original floor out of existing house during renovations?

[OldieNewbie]

Hi all,

 

In a residential renovation, why is the original flooring often taken up and a new sub floor put in?

 

Is it because the original flooring is often taken poor quality or is it to allow for new internal walls and services like UFH to be installed? 
 

Thanks.

[Redbeard]

Probably insulation, primarily. Then of course if you want UFH you are all ready to go.