In variable partition method, in order to have better utilization of main memory, OS must use one of the placement algorithms such as best fit, first fit, next etc.
So if the OS designer chooses to assign memory to processes arbitrarily, it would lead to inefficient use of memory.
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In one of the examples given in the book by stallings, in variable partitioning (referred as dynamic partitioning in the book) :
In this example, processes start by getting allocated one after other (which is not necessary as we'll see later), but later after few swappings have taken place, holes are created. Now OS would have used compaction to line up the free space which is a time consuming problem.
So OS would try to reduce the need of compaction by managing the allocation of holes to processes efficiently and for that uses placement algorithms such as : best fit, first fit, next fit etc. (in our case we are ignoring compaction, which we can think as reducing the need of compaction to maximum extent)
This whole thing, i.e., managing the allocation of holes to processes efficiently can be done from the beginning itself, considering whole memory as one big hole, and use placement algorithms such that the need of compaction is minimized (we can totally ignore compaction in our case)