They need you, but you don't need to say yes. I have done that kind of stuff before, and after a while they keep expecting that kind of "flexibility" from you.
But if at some point you suggest about training your colleagues better, or just hiring more they will tell you there is no budget.
I have like 24 days of extra work I put in going for me at the moment, If Social Welfare ( we call it like that , but in Belgium they just check exploiting of labour) does a sweep at my company.
They will say, hey Huck now goes on a 20 day absence from work ... And I have like 7 irreplaceable colleagues like that, and 40 you can just say we don' t need them.