Certain insect studies may start out by collecting specimens in (large) lots, but sooner or later these specimens tend to be sorted out into lots of size one. In addition, many (millions of?) specimens were and are collected in lots of size one or a few.

Entomologists, especially systematists but also ecologists, observe and record information about single specimens, both in the field and later. E.g., they make observations on host plant and feeding behavior, on mating, on predation/parasitism, time-of-activity, immediate weather and microhabitat conditions, and a long string of other data.

In the lab, individual specimens may be kept alive to rear parasites/hyperparasites, to rear immatures to adults, etc.

In order to associate the observations with the organism that was observed, it makes sense to mark the individual specimen with its unique identifier (or more traditionally, to put some of this information on the printed labels (yes, as small as they are, there is some great data to be had from insect labels!).

Perhaps marine invertebrate collections don't have such individually-focused observations as the norm?

As I noted in an earlier message, insect collections seem to be in the between-world where it's not totally obvious that the individual specimen should be uniquely labeled, and some disciplines where it's quite obvious that lot-labels for (e.g., jars full of) specimens are the way to go. In any case, it is certainly recognized that much data entry for insect collections can be done on "lots" of insects, before they are broken up into single-specimen preps and additional data entered. For the millions of single-mounted specimens from already-broken-up lots, data entry will have to work from the structure it has available now (or in the future when next the specimen(s) are handled).