LOL you got me, I'd like to know the answer to that one too.
They're just fuse kits with the ability to be increased but it's a money trap with boost point loss from each item you put into it instead of fusing direct to bot item.
EXAMPLE I just ran a check on my stuff.
EPIC Power Unit LVL 1 = 3500 Boost Power
ADD another EPIC Power Unit LVL 1 to the first it only increases it +560
making it an EPIC Power Unit LVL 5 = 4060 Boost Power.
SO instead of just boosting a bot item with 2 EPIC Power Unit LVL 1
giving that item 7000 Boost Power you made a Power Unit that only has 4060 Boost Power
wasting 2940 in boost points just to make 1 bigger Power Unit out of 2 Power Units plus the extra SM each time you boost.
Plus keep in mind when fusing items if you use ELE items to fuse into ELE items you get more than if you used EXP or PYS items. Same goes for fusing LEGs to LEGs you get more than if you used a WEAPON on a LEG.
It's not a lot but there is a difference.
So best option when possible fuse
ELE LEG to ELE LEG
EXP LEG to EXP LEG
WEAPONS to WEAPONS - TORSO to TORSO
etc etc etc for each item group to get MAX boost points.