My understanding is that the o-rings are not necessary, but eliminate the occasional need with some cans to tighten the caps with a wrench to prevent any possibility of dripping when stored full on their sides.
This same thing happens with Rotopax cans, which use the same compression design with no o-ring and caps manufactured by Scepter. The solution by Scepter for MWC's and Rotopax for their containers is a "special wrench" to get the caps tight enough, although it has been my suspicion that the Scepter MWC's supplied to the Canadian armed forces may have been fitted with o-rings.
Scepter/LCI can wrench:
View attachment 165382
https://www.amazon.com/dp/B074VH5FQL/?tag=wranglerorg-20
NOTE: The photo above is of a pre-2018 Scepter MWC cap. The design changed slightly in 2018, but the wrenches fit caps of all years, see:
https://jagmte.com/pages/attention-scepter-water-can-owners
Rotopax can wrench:
View attachment 165381
To the best of my knowledge the o-rings are not specified in the U.S. military specifications for MWC's and have never been included with LC Industries MWC's. Reports of leakage are rare even without those o-rings and generally limited to cans stored full on their sides.
It will be great to get confirmation that the o-ring dimensions listed above are correct because I would prefer to use o-rings to prevent any possibility of a leak in the event I decide to use my MWC's on their sides rather than keep track of a "special wrench."