Ethiopia's big goal is to have unfettered access to the sea . Somalia seems like the obvious option because it has the longest coastline, especially becuase it's unstable due to politics and fighting among clans . Both Ethiopia and other countries and entities have common interests, but they each have different and goals.
SL and PL want a deal similar to what Djibouti has: they collect taxes and fees from Ethiopia's trade, and whenever Ethiopia get uppity either up the tariffs or slow down trade. But Ethiopia wants more than just that. It knows that to be a big player in the region or the world, having direct access to the sea is crucial. It wants to outright own and lease the port and the landstrip connecting it to its territory, paying a flat rate and within the leased territory defacto annexing it.
Ocean trade is important for today's economies because ships can move a lot of stuff quickly and cheaply. Messing with major sea routes like the Malacca Strait, Panama Canal, or Suez Canal would upset everyone worldwide because these routes are so essential.
In the end, it's about meeting supply and demand. Ethiopia sees opportunities in Somaliland, Puntland, Djibouti, and even Kenya, though better roads and railways would be needed for the last one. Ethiopia could play Somaliland and Puntland against each other to get a better deal. But what Ethiopia really wants is a long-term agreement and a direct route to global markets.
The more options Ethiopia has, the more it can leverage economic and military prospects to gain the most favourable deal.
This doesn't even touch on the potential worst-case scenario: Annexation.
If there's a disunited response to an annexation attempt of any Somali territroy, it makes annexation a more attractive option. The memorandum of understanding raised popular sentiment across most of Somaliweyn. We've witnessed, especially with the ICU, that even without a formal government, a unified reaction by all Somalis can be extremely perilous to outsiders. Now, with more Somalis govermental entities in political deadlock, it becomes increasingly challenging to muster a collective response to hypothetical Ethiopian aggression.