Practically, because "just letting them in" would probably cause the refugees as many problems as it solved, sadly. "Streaming" them through camps allows a safe roof over their heads, while their needs (medical and physical) are assessed.
For example, on the medical side,one problem that springs to mind is that TB occurrence. While it isn't rife in the ME/NA, is still a problem, so refugees would need to be screened (and if necessary, treated), and it's simpler to do that at the outset, than when they've been in the UK a year or more, and start manifesting symptoms.
On the physical side, it's "easier" to disperse people from a central point to places where there are spaces, than to cross your fingers and hope for some kind of equilibrium. In the end it's about logistics and the need of the state to control.