What is the purpose of the siege the Ukrainian city of Mariupol by the Russian invaders?

What are the geopolitical, military and humanitarian objectives that will be achieved following a successful siege of the civilian population of the Ukrainian city of Mariupol by the Russian military that invaded Ukraine?

Consider also that civilian citizens of Ukraine are not allowed by Putin's forces to leave Mariupol to go to their home country of Ukraine, which essentially traps them with little access to food, water and medicine from the outside.

Note: I have read this post, but it is asking a very different question: Why would countries attack civilian facilities like hospitals during war?

REFERENCES:

Siege of Mariupol
After a Week of Siege, Bloodied Mariupol Plans Mass Graves
Photos: Mariupol residents suffer as Russian forces lay siege
Siege of Mariupol: Fresh Russian attacks throw evacuation into chaos

Answers with references and sources are welcome below. Pure opinions, speculations, and rants are not welcome as per the Help center.