UO Second Age

Does the skill Evaluating Intelligence affect the amount of damage done by summoned creatures, like Energy Vortex or Blade Spirits?

No.

People who rely heavily on summons usually don't take Eval precisely because it doesn't impact the summons damage. They do usually take meditation because the summons for the most part cost a lot of mana (level 8 summons).

what does it do?

increases spell damage outside of summons

Also gives you an idea how much INT your target has, which is key for deciding whether or not to use mindblast against a dexer. And gives you a percentage of mana remaining which gives you an idea how to proceed in a red pot duel.