It is a matter of choice of emphasis, Cambridge says:
Often there is little difference between the past continuous and the
past simple, except that the past continuous suggests that the
event(s) were in progress at a time in the past or that they were
happening as background or temporary events. Whether we choose to use
the past continuous or past simple often depends on how we see the
- Doctors were treating patients in temporary beds and they were
trying to do their best in a difficult situation.
(Past continuous: writer chooses to show the events as ongoing at that
time in the past.)
- Doctors treated patients in temporary beds and they tried to do their
best in a difficult situation.
(Past simple: writer chooses to show the events as finished.)
So in your sentence, it seems the author simply wanted to stress the employment of waiters as a finished action, and as it stands on its own, I think it is a better option.
During the summer more waiters were employed every week.
And it is a better option, because your version would be better off with more context to justify the use of the past continuous. To stress progress at a certain point in time or during a period, it helps the whole statement to connect it to another simultaneous or parallel action in progress:
During the summer, more waiters were being employed every week as the restaurant was attracting more clients.
It is just more plausible to emphasise progress in this way. However, just because it needs more context to make sense, it does not mean it is incorrect.
Here is what Rodney and Pullum says about this
When a clause describes a situation in a way that considers it as a
whole, in its totality, without reference to any internal temporal
structure or subdivision it might have, we say that the clause has a
perfective interpretation. When a clause describes a situation in a
way that makes reference to its internal temporal structure or
subdivisions, we say that the clause has an imperfective
interpretation. The following examples illustrate the distinction :
ia. She wrote a novel.
ib. She was writing a novel.
The natural interpretation of [ia] is
perfective: it simply describes an event that took place in the past.
Example [ib], by contrast, has an imperfective interpretation : we
are not concerned with the total event of her writing a novel, but
with just part of it, some part in the middle during the process of
its composition. Note that it does not follow from [ib] that she ever
actually completed the novel. (English Grammar, p. 42)
A more striking example might help to distinguish the difference:
Several people were injured during the demonstration.
Several people were being injured during the demonstration.