Hi Somi,
TABLE_PLACEMENT is something specific to HANA.Why you hav eto maintain entries in this? there are many reasons to maintain it to give you examples lets assume you have three problems
1) Lets assume in your scale out HANA system one particular node is over utilized and other nodes are less utilized
2) few tables in your system has huge data in it , taht is almost closer to 2 billion rows
3) You feel your current HANA system is full and wanted to add one more node and expand your HANA system.
so in all these cases as a system admin/developer you have to take some action.What is that action is in HANA studio in admin perspective 
user has to
1) Optimize table distribution for problem 1
2) Optimize Table Partitioning for problem 2
3)Redistribute tables after adding host
Ok when you trigger these operation based on what refrence system will do distribution or partitioning?
again to give na example lets assume all the DSO related tables once it reaches 1 billion records then it should get partitioned into 4 parts and should be placed in slave nodes and all cube related table when it recahed 1.5 billion records it should partition into 2 parts and should be placed in master node.
So when you trigger tteh table distribution or partioning how teh system will treat teh DSO and cube tables differently? in order to do this you hav eto mainatin entrie sin TABLE_PLACEMENT entry.
In TABLE_PLACEMENT entry you define teh repartitioning criteria and location for DSO and cube sperately. so then when user trigger the repartitioning or redistribution system will look up this table and does teh operation accordingly.
go through the link http://help.sap.com/saphelp_hanaplatform/helpdata/en/22/888f9344954f258284d2dd936d0d0a/content.htm to understand table placement table better. I hope i have answered your question...