It has no offensive function is probably just reminder text
When, as a DM, the PCs in my campaign encountered an overseer (Lords of Madness 139–40), I'd read the sentence It has no offensive function in the description of the overseer's major creation eye ray as a reminder to the DM rather than rules, the text offering help to the DM wanting to run a very complicated creature.
That's because, as the question implies, the game offers no guidelines on how to judge whether or not a particular major creation effect is somehow objectively offensive. For comparison, while the game offers several different definitions of attack, perhaps only the symbol spells like this one offer an example of what the game may mean by offensive. However, I don't know what conclusion to reach if the symbol definition of offensive is shoehorned into the context of the major creation eye ray. It's really not a good fit.
Still, while the number of ways to employ offensively the 5th-level Sor/Wiz spell major creation [conj] (Player's Handbook 252) are limited largely by the imagination (but see below), the overseer in my campaign didn't use the major creation eye ray in combat with the PCs. I thought that the idea that of such a powerful creature taking a full-round action in combat to use its major creation eye ray paled when compared to, instead, taking a standard action to take the action ready either to render itself immune to a spell via its reactive spell immunity eye ray or to turn a single-target spell back upon its caster via its reactive spell turning eye ray. To this DM, shrugging off or turning back a spell seemed more in line with the behavior of an Intelligence 20 magical creature in a fantasy world where the most powerful force is magic rather than, for example, creating a berm or whatever
I did have the overseer prepare beforehand a pile of mundane weaponry that it used to good effect with its telekinesis eye ray (see here). Just to be clear, though, the major creation spell inherits from the 4th-level Sor/Wiz spell minor creation [conj] (PH 254) the following text:
You must succeed on an appropriate skill check to make a complex item, such as Craft (bowmaking) check to make straight arrow shafts.
(Example absent from the SRD.) And, while a typical overseer does have an Intelligence modifier of +5, a typical overseer has no ranks in any Craft skills. I ruled that its major creation eye ray allowed an overseer to take 20 on a Craft skill check, and, unless a DM rules otherwise—and I don't—anybody not under stress can already take 10 on a Craft skill check, so the overseer could use the major creation eye ray to make before combat weapons for its minions or itself for telekinetic hurling, but making a weapon in combat would be a crap shoot.
However, it just wasn't an option for my overseer to make traps (as suggested by this fine answer to the asker's other question), as I'd not customized the overseer to such a degree that it could create with any degree of consistency meaningful traps. A CR 7–9 trap has a Craft (trapmaking) DC of at least 30 (see here); an overseer is CR 15.
Likewise, an unmodified overseer struggles to make poisons during combat. (This is a common suggested use for spells like minor creation et al.). The overseer continues to rely solely on its high Intelligence score instead of any ranks in a Craft skill. According to Complete Adventurer (98), a d20 roll of 20, for example, might mean an overseer creates a big batch of burnt othur fumes (Dungeon Master's Guide 297 and see here), but even that lucky (?) overseer has no innate immunity or even resistance to poison, so it must exercise a degree of caution that's not necessary when creating most other stuff. Further, any poison it creates may be turned against it if the poison persists for long enough.
Here're a couple of other points to keep in mind:
The Player's Handbook on Conjuration, in part, says
A[n] object brought into being… by a conjuration spell cannot… appear floating in an empty space. It must arrive in an open location on a surface capable of supporting it. The… object must appear within the spell’s range, but it does not have to remain within the range. (172 and here and edited by this writer for relevance)
Thus, as a supernatural ability that largely duplicates a spell of the school of conjuration, typically the results of the major creation eye ray can't appear in the air over an opponent's head.
To strike a weapon or shield that an opponent's wielding requires the creature to make the special attack sunder (PH 158), and a sunder attempt requires a melee attack rather than, for example, using telekinesis to hurl an adamantine heavy mace at a spearman's favorite longspear. That's not to say there aren't ways to make sunder attempts at range (q.v. obviously the feat Ranged Sunder (Complete Warrior 104)), but a customized overseer would usually benefit more from using its feats to improve its eye rays or something.
In short, unless the overseer knows that the PCs are totally mundane so that it won't need spell immunity or spell turning, the overseer's major creation eye ray will likely rarely be of use during combat. (An obstacle may occasionally be useful, I guess.) To this reader, the major creation eye ray's limited utility only helps reinforce the idea that Lords of Madness is just being helpful by reminding the reader that the overseer's major creation eye ray "has no offensive function."
Note: I thought for sure that D&D 3.5 increased the Craft skill check DC when the craftsman is working with adamantine, but I couldn't find rules that supported that notion. I know that the Craft skill checks in Pathfinder work that way.