This was mentioned earlier, but when you Sub Cards that have the same Skill Icon as your Main Card, the Main Card gets an extra Parameter Boost, giving you more Stats. Not surprisingly, this not only makes your Main Card that much stronger, but it still adds to your overall HP and Defense. The main point here is at what point do you favor an extra Parameter Boost vs a high Skill Boost? The idea is that you don't. You always want to be aiming for the best Skill Boosts whenever necessary. If the Skill Icon matches, that's even better!
For example, let's say you're choosing whether to Sub [[Walrus]] vs [[Zenon]] on a Stab Card. Walrus obviously works well on a Stab card, but you need a Straight+ to activate his Sub. Zenon on the other hand works for any Attack Card and is Unconditional to boot! Zenon in this case makes for a better reliable Sub, even if it means you have to sacrifice the added Parameter Boost.
**Stab: Robin Hood, Tsubame (College), Jack the Ripper