Basically, they are expecting that the price will grow as they sit idly on their hands. But this growth is not possible without other people actively using bitcoin, paying with it and selling for it - all those people who help create real value of bitcoin. And I know what hodlers are going to say in reply. That they diminish the supply and thus also contribute to this growth.
Your point is valid in some way but I dont think saying parasites to the hodlers is justifiable as everyone cannot do trading (may be due to lack of knowledge of TA or time constraints or some other reasons) so they want to be part of this space (for the sake of money may be not the tech behind it and to be frank most of the people here are for money if it was not like that then we would not have been seeing bitcoin at this level).
And for the buying with bitcoin part let me tell you bitcoin is not yet globally traded as currency for everyone to use it as an alternative of fiat currency to buy the necessities.