Rogue Universes

Some think that only species engage in combat and growing their empires however, this is incorrect as some of the actual physical universes engage in these sort of practices too with them known as rogue universes due to them absorbing other universes to grow bigger like the species they house in a way.

Ascension
Through absorbing countless universes, it is theoretically possible for a universe to ascend to a higher power level aka the dimension manipulator class which thus, gives them a massive power boost in what they can do along with changing the dynamics of these universes dramatically due to universes typically being in the universe-class.

Through history, there have been some recorded incidences of ascensions however, the vast majority of universes don't participate in such practices like absorbing other universes and the vast majority of the ones that do never reach the next class up due to the massive time and universes needed to absorb.

It is said that the vast majority of ascended universes tend to be around the location of a high-end dimension monarch who in turn is said to be the most powerful beings in his class and is someone who in turn goes around absorbing the superclusters of other dimension monarchs.

Motivation
There are several reasons why a universe would start devouring other universes and here's a list of some of these reasons:
 * Rogue Universes: Due to their being rogue universes in the area, a universe may start absorbing other universes to become powerful enough to fend these other universes off rather than, being absorbed by them.
 * Powerful Foes: A universe may start absorbing other universes due to there being beings such as universe eaters that they wish to have a defense against.
 * Powerful Residents: In this case, it means that the inhabitants within the universe have become too powerful and so, threaten the balance of the universe in some way which leads the universe to absorb other universes to overpower these residents.
 * Orders from Above: A dimension lord or someone who this universe falls under may have given an order for the universes to become more powerful by absorbing the weaker universes which could happen due to a number of reasons such as gathering an army or to house powerful beings without exhibiting fatigue.