Magmasaurs are excellent in collecting stone, crystal, and metal. Metal receives a very notable weight reduction. They can do full damage to rock golems as well. X-golems taking less damage as an X creature.
Their smelting is lackluster, very slow for the incredible amount of food it uses to smelt. Stick to a pheonix that does so passively, unless you're in the lunar biome for ambergris to keep them fed.
Having one makes getting eggs easy, but it is still easier to shotgun the wild ones then get the eggs, if you've a decent scattergun.
Magmasaur is able to destroy ALL STRUCTURES utilizing its searing spit. While it is a walking forge it produces metal at the same rate as a refining forge. You can use the jump button to taunt other creatures and when holding searing spit use crouch to erupt. (This attack pushes creatures back and leaves magma everywhere) it is breedable in lava. Hope this helped