In formats like Pauper, where all dual lands are tapped lands, they are one of the best land cycles in the format, with a whole deck (Skerd) being built around them
For the simple cost of two Colorless energies, which could be instantly fulfilled in a single turn using a Double Colorless Energy, Seismitoad dealt a modest 30 damage, also applying a status condition that makes it so the opponent can not play any Item cards from their hand during their next turn
It is extremely difficult to get a CGC 10 and once we see their population report, I think the prices of 9.5s from CGC will see a boost
Lifegain effects fall into two categories: single-effect, big bursts of lifegain from cards like Feed the Clan, and incidental lifegain from cards like Soul Warden and Kambal, Consul of Allocation
If the creature you control is an illegal target, you won't put a +1/+1 counter on it even if it's still on the battlefield