During the Theros block, Heroic decks were very popular. The Heroic mechanic depended on spells targeting a creature to activate their Heroic abilities. Spells with Strive were particularly relevant for these decks. Most Heroic creatures already would gain a +1/+1 counter upon being targeted. So gaining an additional boost and being able to tap to fight another creature can be quite handy, as well.
It's a pretty simple combat trick, but Setessan Tactics offered a very inexpensive way to trigger Heroic while getting a good deal of value, as well. It's one of those cards that the later in the game that you cast it, the better it will usually be. The printing of this card seemed to make a good deal more Heroic creatures playable. However, Setessan Tactics never worked its way into competitive Heroic decks. It did, however, see a fair amount of Standard play in sideboards alongside a card that wasn't even from the Theros sets.
It turned out that Setessan Tactics worked very well with a creature called Hornet Nest in a rather clever way. By allowing the Nest to be able to fight a creature, you could get a bunch of flying tokens with deathtouch. For example, say your opponent had a Stormbreath Dragon, an extremely popular creature at the time. You would be able to get 4 hornet tokens by tapping the nest to fight the dragon.
Another trick people would use involved Aspect of Hydra, a spell popular in mono-Green Devotion decks at the time. You would cast the Aspect of Hydra on the creature you intended to fight. Considering that Aspect of Hydra considers the green mana symbols in permanents you control (which would be a lot in that deck), you could end up making a ton of tokens, while assigning a token to block the creature you were trying to deal with anyway.
Alternatively, you could have it fight one of your own big creatures, giving you a bunch of tokens to attack with on the next turn. Green decks in Standard at the timehad some big creatures, including Polukranos, World Eater and later Dragonlord Atarka. While the Setessan Tactics & Hornet Nests were usually relegated to the sideboard, it definitely was a solid way for Mono-Green Devotion and Selesyna (Green & White) Aggro decks to steal a game out of nowhere.
Setessan Tactics is a great little card that probably could have seen a lot more competitive play than it did. It's actually a Modern playable card, and I'm a bit surprised that we haven't seen it more than we have.