I was wondering why some airfields, for example Tatenhill Airfield (EGBM), don't have anybody in their ATC towers?
In the case of Tatenhill, the field was previously used by the RAF. Evidently the original control tower was replaced by a previous operator, which explains why there is still a control tower there. But currently it is a private field, where the amount of traffic doesn't warrant operating the control tower.
There are many airports in similar situations, that do not operate a control tower (whether or not they have one), or only operate at certain times, when there is not enough traffic for a control tower to be needed. VFR rules give pilots the responsibility to "see and avoid" each other in these areas, and announce their intentions on the appropriate frequencies.
Also, see the related questions to the right, especially: What's the process for an uncontrolled airfield to become controlled/towered?
One of the biggest reasons to not man a tower is (not unsurprisingly) costs.
Many remote airports only have a few movements per day, and a permanent staff in the tower makes it even more difficult keep the operation economically viable.
For this reason, remotely staffed towers are being developed now, and for instance Örnsköldsvik airport (ESNO) in Sweden has just received approval from the Swedish Transport Agency to operate with a remote tower:
The system’s sensors are installed at Ornskoldsvik Airport but traffic is remotely controlled from Sundsvall, Sweden.