Fiber glass corners not recommended?

Came across a few threads with guys saying US composites does not recommend fiber glassing corners under the epoxy, just wondering your thoughts on this. So if one was to not glass the corners what would you do to completely seal the corners? Fillets?

Ah yes, there will be a space for sure.
How about fillets but no mesh?

Correct me as I may be wrong but, fiber mesh and resin is doing what: Strengthening the corners, right?
Or (and) sealing corners from water.

Now if a tank is built very strong with well reinforce corners, what's really the need for the mesh? What I'm thinking is plug the corners ( Fillets) then just coat a couple times with Resin!

What I would like to know what does the mesh do either then strengthen and seal?