That's a flattering HEV suit you got there

(Resistance giggling in the back)

*lip trembling* pwease i- *sobbing*

The white-and-blue concept art for Chell and that one Mac promo Gordon look funky to me..

Originals under the cut

Earth's Martyr.

The hips are stable and don't typically have extra padding, both in older and more modern armor (regardless of region). There are tassets, which is armor meant to protect the upper thighs. The tassets are usually longer than Gordon's lil hip plates. They often attach to a fauld, which protect the waist and hips (mainly the waist, since it's a squishy area with important stuff in it). But they don't take the form of individual hip plates, they're just a band of lames around that general area.

(1450's milanese armor from the Karnsten Kingbeil Collection. Not the highest quality image, but you can see the tassets that connect to the faulds)

In older versions of the HEV suit, Gordon has a breastplate that's cut off around the lower ribs. The material that follows this breastplate is sometimes depicted as being riveted (overlapping lames that are weaker than normal armor but provide more flexibility). The riveted middle piece then attaches to what I can only call diaper armor.

It seems to be a small, belt-like fauld with a codpiece. The fauld is highwaisted enough to where it could be protecting the waist, but it's more thin than typical faulds and looks more like a tool belt. The codpiece is also given 3d dimension in the Dreamcast version of HL1. The riveted armor continues down the sides, not providing a lot of protection for the upper thighs. The riveted armor then attaches to cuisses, which protect the thigh.

(Collection of Mark IV HEV suits made by u/throwawayasdf129560 on r/HalfLife. Note the codpiece on the Dreamcast version)

So one could argue the slut waist pieces are purely for protection. Maybe not great protection, but protection regardless.

However, you could also argue that (in latter depictions of the HEV suit) the hip pieces are for accentuating the slut waist.

A trend in medieval fashion, both in and out of armor, was to have extravagant codpieces, especially for decorative armor. This would be done by increasing their length, guilding them, and even adding lil faces to them. There is nothing advantageous about having a big codpiece. In armor worn for combat, they were not nearly as exaggerated. This is purely done to show off and brag about one's virility, in a classy aristocratic way.

(1550-1560 armor belonging to Kaspar von Montani from the Armoury of the Castle of Churburg)

The orange pieces of the HEV suit seem to be more protected. The HL1 armor's orange segments make sense. They might not be perfectly optimized, but all the places with extra protection could plausibly need them. The HL2 armor's orange (and extra protective arm pieces) make sense for the most part. Until you get to the tiny hip pieces. This is probably just a leftover from the HL1 armor, and it's a fun detail that makes the armor more visually interesting. But practically, it has no purpose.