Jasmin,
You'll just have to accept therefore that your logic on this occasion lets you down.
In any case what's the relevance of services and work even aside from the fact that of course services are work?
The question is about retaining something, and as you've been told there is nothing strange or illogical about retaining the services of someone else. Or in other words you are securing the right to call on that person to perform work or a service for you on some future occasion.
Just believe me.
Regards