Because Delver of Secrets and Huntmaster of the Fells have mana values of 1 and 4 respectively, so do their transformed versions, Insectile Aberration and Ravager of the Fells
You get a more powerful Elemental Bond effect, also adding +1/+1 counters to small tokens
She doesnt tap for mana or find lands, she just makes all your big creatures (power 4 or greater) cost 2 generic mana or less to cast
If you do, attacking creatures get +X/+X until end of turn, where X is the number of time counters among permanents you control
Ayula, Queen Among Bears Beorn the Fierce Beorn, Reluctant Host Bosco, Just a Bear Doc Aurlock, Grizzled Genius Duskana, the Rage Mother Goreclaw, Terror of Qal Sisma Hei Bai, Forest Guardian Hei Bai, Spirit of Balance Hurska Sweet-Tooth Kudo, King Among Bears Lumra, Bellow of the Woods Ruxa, Patient Professor Surrak and Goreclaw Wilson, Refined Grizzly
Thats just a whole lot of stats and abilities on a remarkably cheap creature