Filling with foam to reinforce the tubing, reduces that, increases overall strength as well, allowing for fewer, or possibly no cabling, while reducing drag due to cabling. Foam is generally lighter than aluminum and steel.
The Albatross was a minimum mass cable braced design. So in that case no, foam is just dead weight.
But that's an edge case! Will filling a fiberglass or carbon fiber tube with foam make it stronger? Eh. Meh.
It will help hold the broken parts in loose formation, but the foam takes zero strain in any normal sense. The perimeter of composite takes all the load until it fails, and the foam is useless. Sure it's lighter than rock or iron, but so are soap bubbles. Just a waste of money and mass.
Where foam comes into it's own is shaping like ribs under fabric, or even a Rutan full core composite moldless construction. You'd get lighter structure if you used molds and infusion to better optimize resin /fiber ratio.
But that's a LOT of work. Mold-less foam core can be done in a basement or garage with a leveled table and hand tools. There's a lot of hand sanding but making molds takes that too.
The owner/builder of Lionheart said he sanded and filled the fuselage molds until he could see the reflection of the mountains outside his shop without distortion. That's more hand work than a typical Rutan design. He was rewarded by a lovely result, though.
Mike Arnold's videos are great. And the best advice besides those seems to be from the designer of Symmetry. Do each step once, as perfect as you can, so the next takes less work. He put extra effort into making the foam smooth, so irregularities didn't have to be sanded off the composite. He put extra effort into making the layup even, so the finish layer of micro didn't have unnecessary lumps to sand off, & layed one thick enough smooth layer of micro so the final sanding to shape within hundredths was a one time job.
Wish I'd heard that advice before my first lousy glass job.