Ouroboroid turns its own power into permanent growth for your entire creature board. At the beginning of combat on your turn, it puts X +1/+1 counters on each creature you control, where X is its power. Since that includes Ouroboroid, the first trigger can make the next trigger much larger if the creature survives.
There is an immediate opportunity in the casting turn. Play it before your beginning-of-combat step and it can supply counters even though it normally cannot attack yet. Existing attackers receive the increase before attackers are declared, letting a four-mana creature change combat without haste. Playing it after that step misses the ordinary trigger for that combat.
Counter support such as Branching Evolution increases the counters placed, while a power boost applied before the trigger resolves can increase the value of X. Extra combats create additional beginning-of-combat steps on your turn, allowing repeated triggers to build on the counters already present. The resulting growth is lasting, unlike a temporary bonus that disappears at end of turn.
The card rewards both width and a strong central creature. A single Ouroboroid can grow itself, but a board of tokens receives much more total power from the same trigger. Trample from Kodama of the West Tree helps that power reach players, and protection helps preserve the source of the next large counter event. If Ouroboroid leaves before its trigger resolves, the ability uses its power as it last existed on the battlefield. Removing the creature does not simply erase the counters its pending trigger can supply.
