axle truss question

So I have been looking at trusses people put on in some of the builds. I am curious about putting them over or under? I know a truss on top keeps the clearance as high as possible but then the first thing to hit anything is the axle tube. From a physics perspective I am curious if the axle tube is supposed to be strong enough to take a hit from a rock why isn't it strong enough to not have a truss at all.

I know a truss on the bottom takes away some clearance but then it would be protecting the axle. Also would it not also allow more up-travel than an over-the-axle truss?

Someone school me in trusses please.

a truss is not meant to protect the axle housing, but rather... whats inside. the entire housing, while it seems rigid, can deflect and flex. this deflection is a big reason for ring and pinion failures.

I thought it was for rigidity of the axle tube. How much flex can you have before seals and shafts get messed up?

Seals and shafts aren't the issue, cracking a housing is!

Not to mention tubes can break out of the center section. Axle tubes can have a surprising amount of flex in them in stock form. Adding a truss is kinda like triangulating a roll cage and properly designed makes the entire thing stronger

so...a truss is basically holding the tubes to the center section? and limiting some flex?