They can pump each other's stats up, tutor for other Allies, or activate specific effects if one enters
As with most things in this format, it's not simple
But sellers may be betting that well see fewer reprints of them in the future if they become semi-officially mandatory to access the highest-powered games in the format
Now the only way an opponent could evade infect damage would be to always have a chunk blocker ready, exile Plague Myr, or enchant it with a Pacifism-type aura to prevent it from attacking and blocking
When Pit of Offerings comes into play, you get to exile up to three cards from any graveyard, getting rid of whatever the biggest threat in the yard
Anything that counts your number of lands or permanents, such as the Citys Blessing is going to be very consistent for us