Before starting, no I didn't forget the white one; there is no white tribal booster. I don't know why.
Warriors are quite prevalent in Magic and their usually combat-based, so giving them a combat boost is great. However, the nicest thing about this card is that it gives all your creatures with a +1/+1 counter trample. For comparison purposes, Crowned Ceratok costs 4. Yeah, that card is a 4/3 with trample, but it doesn't boost other creatures that you play.
Pros: Gives trample to boosted creatures
Cons: Just a bear
Rogues, while not as combat-oriented as Warriors, usually have some form of evasion, so giving them an ability that triggers on combat damage is great. Also, since rogues are traditionally small, so you'll get a fair bit of mileage out of the ability.
Pros: Forces opponents to discard when hit by boosted creatures
Cons: Unfortunately, it's a Bog Imp
This is the old duck in this group since it only boosts when it comes into play, but that was necessary to keep it from being broken. Shamans, while not exclusively combat focused, can easily overwhelm an opponent in red since they tend to be really cheap, so the constant pinging can easily add up.
Pros: Turns boosted creatures into pingers
Cons: Simply a Goblin Hero, only boosts when it comes in
Sage of Fables
Wizards are the least combat focused of all of these creature types, but that's why they didn't give them a combat effect. The Instant speed, colourless draw is really useful in blue and giving it to every Wizard you control is sweet.
Pros: Colourless draw off of boosted creatures
Cons: Only a Gray Ogre