A Useful Card for Any Magic the Gathering Deck Builder's Toolbox
In Mono-Green decks, which were quite popular at the time, you could make a powerful Trampling fatty creature like Vorapede so big that your opponent could easy take lethal damage from it. Since Green has so many ways to Trample, that extra power and toughness could often bash right through for the win.
But there were other things to do with the counters that excited people. Fathom Mage was a card people really liked. Even though it's a 1/1, it can draw you a card each time a +1/+1 counter is placed on it. It would trigger for each counter placed on it. That basically means 5 cards for 4 mana, and a 6/6 Mage. That seems like a good combo. But in practice, the Mage was a bit too fragile to be worth casting in Standard for the most part.
The other creature that people really wanted to try with Increasing Savagery was Gyre Sage. A creature with Evolve that did see a fair amount of Standard play, the Sage could tap for one Green mana for every +1/+1 counter on her. With Increasing Savagery, you could net positive every time by essentially being able to tap for 5 mana right after paying 4. It's a good combo
In theory, this is a great card. In Standard at the time, some players felt that Thrun, the Last Troll might be a good target. It turns out that some people did in fact find the Hexproof troll a good use of this Savage sorcery. Increasing Savagery did see some top level Standard play, however, in Bant Hexproof decks such as this one that placed 3rd at Grand Prix Kitakyushu 2013. Other strong decks that played one or two copies of Increasing Savagery include this Golgari Aggro brew from a StarCity Games Super IQ in Littleton, CO and Gruul Aggro decks that performed well on Magic Online like this one.
While Increasing Savagery has never lit any Constructed formats on fire, it's been a key part of some strategies in the past. Certainly, it's a good casual card. Some very good Commander decks, such as Vorel of the Hull Clade, can make good use of it. Skullbriar, the Walking Grave is particularly a fan of this card, due to being a Commander that can keep its +1/+1 counters even when it's not on the field! Also, while not a staple in Animar, Soul of Elements decks, Increasing Savagery can give Animar quite a boost, which in turn makes it a lot easier to cast your creatures!