I'm sure the clue will come!

In fact, the Summoner is the maximum level of playable cards.
With a level 8 summoner, you can play all cards at the maximum level.
The higher the level, the stronger the cards.

Let's take VENARI CRYSTALSMITH as an example.
It is significantly stronger at level 8 than at level 1.

That's why a good balance is important. The (expensive) Summoner is not much use if the cards are not leveled up accordingly.
Conversely, cards at level 8 are of no use if the Summoner at level 1 downgrades this card to level 1 for combat.