Ferritin Sac has 1 very clear purpose, spending red to get blue. It can’t act as a blocker, it can’t generate attack, all it can do is die to generate a blue. Unfortunately it’s 2 turn construction time can make it pretty inefficient as a replacement for your first Blastforge, but it can be pretty useful for units that cost more than 1 blue and have very limited supply.
Low Supply 2+ blue units
A set lacking many blue defenders to use multiple Blastforges on could really benefit from being able to simple use 2 Ferritin Sac’s to augment your single Blastforge and pick up the single supply defense grid.
Energy Matrix benefits from Ferritin Sac in low blue sets for the same reason as Defense Grid. Even with 4 supply of Energy Matrix and no Blastforge, there’s 10 supply of Ferritin to be able to cover the 8 blue you’d need.
Unfortunately you can’t benefit from Grenade Mech’s click ability without Blastforges to consume, but at least Ferritin Sac can provide a backup source of blue for situations when you need to consume all your Blastforges. Thankfully just like the Pixies, the Ferritin Sac’s click ability can wait until you need it.
Ferritin’s cheap cost can make it easier to grab 1 or more of these earlier than a potentially unnecessary triple Blastforge, giving you a good attack or defense advantage early.
The rainbow tech requirements can make it even slower to get one of these up for defense. With Ferritin Sac in the set, it can be much easier to build a Centurion with an early Animus/Conduit.
Getting an early Apollo can really put a damper on your opponents tempo. Now its much easier to turn an early Animus/Blastforge into an Apollo by just picking up 2 Ferritin Sac’s for 2 gold and 2 red.
Getting first Arka can be pretty important. Ferritin Sac can make it easier to rush Arka and still focus on red.
Now that Wincer was reduced slightly in cost, that 1 gold reduction can be combined with a spare red to pick up the extra blue needed via a Ferritin Sac now!