Because proliferatelink lets you proliferate before checking for lethal damage on creatures, Phyrexian Seedling (and other creatures you control with +1/+1 counters on them) may be able to survive combat damage that would otherwise be lethal. For example, if a Phyrexian Seedling with one +1/+1 counter on it attacks and is blocked by a 1/1 creature, it would deal 1 damage to that creature and take 1 damage from that creature. Before checking for lethal damage, the Seedling’s controller would proliferate one time and could choose give the Seedling another +1/+1 counter. Then when state-based actions are checked, the Seedling would be a 2/2 with 1 damage marked on it and would survive.