Overview & Obtaining
Pants are the one and only true form legwear in RimWorld - slightly protecting one's legs from wear and tear, insulating slightly, and contributing to protecting one's mind from the thought of being bare-skinned (to which this is all a male needs, but a female needs something to cover their upper-half). Pants can be purchased from traders, crafted, or stripped from Outlanders and Pirates.
As a complicated garment, Pants can only be made at tailoring benches, which requires complex clothing to be researched in order to be constructed. A pair of pants requires 50 of any textile, and 117 seconds of work (7,000 ticks) to be created, assuming baseline global work speed, consciousness, sight, and manipulation.
Conclusion & Comparison
Pants can't truly be compared to any other garment as they're the only in their class. The closest piece of apparel that comes to pants is tribalwear, which insulates slightly better (10 Celsius vs. 7 for Pants w/ Button-down shirt). On that note; Tribalwear provides absolutely no protection (unless made from materials such as devilstrand}, and armor vests can't be worn on top of it as Tribalwear occupies the on-skin and middle slots.
The following table(s) will display how good some normal quality Pants are at insulating when made from various materials...